Every IMG who has gone through a stack of USMLE immunology questions knows the feeling. You memorize ten different primary immunodeficiency diseases, their inheritance patterns, and the associated infections, and the moment the NBME hands you a vignette about a six month old boy with oral thrush and Pneumocystis pneumonia, you freeze. Was that SCID? Was that DiGeorge syndrome? The answer seemed obvious in the table you reviewed that morning, but the clinical stem changed just enough to blur the lines.
That confusion is not a knowledge gap. It is a pattern recognition gap. Immunodeficiency disorders on Step 1 are rarely tested as isolated recall questions. Instead, the NBME layers microbiology over genetics, pediatric presentation over laboratory values, and USMLE pathology integration over pharmacology. The student who memorized the table but never learned to read the vignette clues will consistently pick the wrong answer. These are core NBME immunology concepts that require clinical reasoning, not just recall.
This resource was built for that exact problem. At IMG Helping Hands, the UIT program teaches immunodeficiency disorders through mechanism based pattern recognition rather than table memorization. Think of this as your USMLE immunology notes built for clinical reasoning. Every disease here is broken down by what fails in the immune system, what infections result, what the lab shows, and how NBME specifically tests it. By the time you finish reading, you will not just know these disorders. You will recognize them on sight, making Step 1 immunology feel straightforward rather than overwhelming.
| UIT Teaching Pearl The fastest way to solve an immunodeficiency question is to ask three things in order: 1. What type of infections does the patient get? (Bacterial = B cell or phagocyte. Viral/fungal = T cell. Neisseria = complement.) 2. What age did symptoms start? (Less than 6 months = T cell or combined. After 6 months = B cell as maternal IgG wanes.) 3. What does the lab show? (Low immunoglobulins = B cell. Low T cells on flow cytometry = T cell. Normal labs with recurrent abscesses = phagocyte.) Master this triage and you eliminate 2 to 3 wrong answers before you even think about the specific disorder. |
This table covers the core primary immunodeficiency diseases tested on USMLE Step 1. Bookmark it. Return to it before every practice block. The buzzwords column alone can save you on exam day. These are the high yield immunology associations that NBME tests repeatedly.
| Disorder | Immune Defect | Key Infection | Inheritance | Lab Findings | Buzzword |
| Bruton Agammaglobulinemia | BTK gene, no mature B cells | Encapsulated bacteria (S. pneumoniae, H. influenzae) | X linked recessive | Absent immunoglobulins, no B cells on flow | Boy, recurrent sinopulmonary infections after 6 months |
| Selective IgA Deficiency | Low IgA, other Ig normal | Giardia, sinopulmonary | Variable | Low IgA, normal IgG and IgM | Anaphylaxis to blood transfusion |
| CVID | Defective B cell differentiation | Encapsulated bacteria, Giardia | Variable | Low all immunoglobulins, normal B cell count | Adult onset recurrent infections, increased autoimmune risk |
| Hyper IgM Syndrome | Defective CD40L on T cells | Pneumocystis, Cryptosporidium | X linked recessive | Elevated IgM, low IgG IgA IgE | Opportunistic infections with high IgM |
| DiGeorge Syndrome | 22q11.2 deletion, absent thymus | Viral, fungal, Pneumocystis | Autosomal dominant (de novo) | Low T cells, low PTH, absent thymic shadow | Tetany, cardiac defects, abnormal facies |
| SCID | Multiple gene defects (IL2RG, ADA, RAG) | All opportunistic organisms | X linked or autosomal recessive | Absent T cells, low or absent B/NK cells | Failure to thrive, absent thymic shadow, early death without transplant |
| Wiskott Aldrich | WASP gene mutation | Encapsulated bacteria, opportunistic | X linked recessive | Low IgM, elevated IgA and IgE, thrombocytopenia | Eczema, thrombocytopenia, recurrent infections (triad) |
| Ataxia Telangiectasia | ATM gene, defective DNA repair | Sinopulmonary infections | Autosomal recessive | Low IgA, elevated AFP | Cerebellar ataxia with spider angiomas on face |
| Hyper IgE Syndrome | STAT3 mutation (Job Syndrome) | Staphylococcal abscesses | Autosomal dominant | Elevated IgE (more than 2000), eosinophilia | Cold abscesses, coarse facies, retained primary teeth |
| CGD | NADPH oxidase defect | Catalase positive organisms | X linked recessive (most common) | Negative nitroblue tetrazolium test, abnormal dihydrorhodamine | Recurrent abscesses with granuloma formation |
| LAD Type 1 | CD18 integrin defect | Skin and mucosal bacteria | Autosomal recessive | Elevated WBC, absent pus despite infection | Delayed umbilical cord separation |
| Chediak Higashi | LYST gene, defective microtubule function | Staphylococci, streptococci | Autosomal recessive | Giant granules in neutrophils on smear | Partial albinism, peripheral neuropathy |
| C5 to C9 Deficiency | Defective MAC formation | Neisseria meningitidis and gonorrhoeae | Autosomal recessive | Low CH50 | Recurrent Neisseria infections in teenager |
| C1 Esterase Inhibitor Deficiency | Unregulated complement activation | Not infection related | Autosomal dominant | Low C4 (screening), low C1INH | Hereditary angioedema, episodic facial and laryngeal swelling |
| DAF Deficiency (PNH) | Absent GPI anchored proteins (CD55, CD59) | Not primarily infection related | Acquired somatic mutation | Low haptoglobin, elevated LDH, positive flow cytometry for CD55/CD59 loss | Hemolytic anemia, pancytopenia, venous thrombosis |
Immunodeficiency disorders USMLE questions appear on virtually every NBME practice exam. They are tested not as standalone recall items but as integrated clinical vignettes that cross immunology, microbiology, genetics, pediatrics, and pathology. A single question might describe a six month old boy with failure to thrive, oral thrush, and Pneumocystis pneumonia, then ask you to identify the defective gene, the inheritance pattern, or the appropriate diagnostic test. Understanding these NBME immunology concepts at the mechanism level is what separates high scorers from everyone else.
The NBME also uses these disorders to test pharmacology indirectly. You might be asked about the treatment for chronic granulomatous disease (interferon gamma) or about why live vaccines are contraindicated in SCID. Knowing the disorder alone is not enough. You have to understand the mechanism deeply enough to predict what the exam can ask about it from any angle. This is where USMLE pathology integration becomes essential.
Pediatric presentation patterns are another layer. B cell deficiency disorders typically present after six months of age when maternal IgG wanes. T cell deficiency disorders and combined immunodeficiency present earlier, often within the first few weeks of life. The NBME uses age as a major differentiating clue, and students who miss this pattern lose easy points. Recurrent infections on USMLE questions always carry hidden clues in the age, the organism, and the lab values.
| UIT Teaching Pearl: Recurrent Infections USMLE Pattern Recognition When you see an NBME question about recurrent infections, immediately categorize by organism type:Encapsulated bacteria (pneumococcus, Haemophilus) = B cell deficiency disorder or complement defectViral and fungal (Candida, CMV, Pneumocystis) = T cell deficiency disorderCatalase positive organisms (Staph aureus, Aspergillus, Serratia) = Phagocyte dysfunction disease (think CGD)Neisseria specifically = Terminal complement deficiency (C5 to C9)This single framework for opportunistic infections in immunology eliminates most wrong answers instantly. |
B cell deficiency disorders result from defective antibody production. Because maternal IgG crosses the placenta and protects the newborn for roughly six months, B cell deficiency disorders typically present after this protective window closes. The hallmark is recurrent sinopulmonary infections with encapsulated bacteria. Labs show low or absent immunoglobulins. Recognizing this pattern is essential for USMLE immunology questions.
Defective Component: Bruton tyrosine kinase (BTK), a cytoplasmic tyrosine kinase essential for pre B cell maturation into mature B cells.
Pathophysiology: Without BTK signaling, B cell development arrests at the pre B cell stage in the bone marrow. No mature B cells enter the periphery. No plasma cells form. No immunoglobulins of any class are produced.
Genetic Mutation: BTK gene on Xq21.3.
Inheritance: X linked recessive. Affects boys almost exclusively. Carrier mothers are asymptomatic.
Clinical Presentation: Boys present after 6 months of age with recurrent otitis media, sinusitis, pneumonia, and bronchitis. Absent or very small tonsils and lymph nodes on exam. No palpable lymphoid tissue. Recurrent bacterial infections are the dominant clinical feature.
Most Common Infections: Streptococcus pneumoniae, Haemophilus influenzae type b, Staphylococcus aureus, Pseudomonas (in severe cases), Giardia lamblia (intestinal).Lab Findings: All immunoglobulin classes are absent or profoundly low. Flow cytometry shows absent CD19+ and CD20+ B cells. T cell counts and function are normal.
Diagnostic Test: Flow cytometry for absent B cells. Quantitative immunoglobulins showing panhypogammaglobulinemia. BTK gene sequencing confirms diagnosis.
Pathology Correlation: Lymph node biopsy shows absent germinal centers. Bone marrow shows maturation arrest at the pre B cell stage.
Pharmacology Correlation: Lifelong IVIG replacement therapy. Aggressive antibiotic treatment for infections. Live vaccines are generally avoided.
NBME Buzzwords: Boy after 6 months, absent tonsils, no palpable lymph nodes, panhypogammaglobulinemia, recurrent sinopulmonary infections.
Common Exam Trap: NBME may describe a boy with recurrent infections and ask why maternal antibodies no longer protect him. The answer is always that maternal IgG has a half life of about 21 days and is depleted by 6 months.
High Yield Mnemonic: “Bruton = Boys, B cells Blocked, Bacteria Battle.”
One Line Summary: X linked BTK mutation causes pre B cell arrest leading to absent B cells and panhypogammaglobulinemia presenting after 6 months with recurrent bacterial infections.
Defective Component: IgA producing B cells fail to mature. Exact mechanism is not fully defined but involves defective isotype switching to IgA.
Pathophysiology: IgA normally protects mucosal surfaces (respiratory, GI, urogenital). Without adequate IgA, mucosal defenses weaken, leading to recurrent sinopulmonary and GI infections. Other immunoglobulin classes remain intact.
Inheritance: Most cases are sporadic. Some familial clustering with autosomal inheritance patterns.
Clinical Presentation: Many patients are asymptomatic. Symptomatic patients develop recurrent sinusitis, otitis media, diarrhea, and increased susceptibility to Giardia. Strong association with celiac disease and other autoimmune conditions. The most dangerous complication is anaphylaxis during blood transfusion due to anti IgA antibodies.
Lab Findings: Serum IgA less than 7 mg/dL. IgG and IgM are normal. B cell counts normal.
NBME Buzzwords: Anaphylaxis after blood transfusion, recurrent diarrhea (think Giardia), celiac disease association, most common primary immunodeficiency.
Common Exam Trap: The NBME loves to ask about the transfusion reaction. If a patient receives blood products containing IgA and has anti IgA antibodies, anaphylaxis results. The treatment is to use washed RBCs or blood from IgA deficient donors.
One Line Summary: Most common primary immunodeficiency, characterized by low IgA with intact other classes, presenting with mucosal infections and risk of anaphylactic transfusion reactions.
Defective Component: Defective B cell differentiation into plasma cells. B cells are present but do not produce adequate immunoglobulins.
Pathophysiology: B cells are present in normal numbers but fail to undergo terminal differentiation and class switching. This results in low levels of IgG, IgA, and often IgE. The underlying defect is heterogeneous and may involve intrinsic B cell defects or defective T cell help.
Inheritance: Variable. Most cases are sporadic. Some are associated with ICOS, TACI, or BAFF receptor mutations.
Clinical Presentation: Typically presents in the second or third decade of life. Recurrent sinopulmonary infections, chronic diarrhea (Giardia), bronchiectasis, and significantly increased risk of autoimmune disease (autoimmune hemolytic anemia, ITP) and lymphoma.
Lab Findings: Low IgG, low IgA, often low IgE. Normal or near normal B cell count on flow cytometry. This distinguishes CVID from Bruton where B cells are absent.
NBME Buzzwords: Adult onset hypogammaglobulinemia, normal B cell count, autoimmune complications, lymphoma risk.
High Yield Mnemonic: “CVID = Common, Variable, Immunoglobulins Down but B cells still around.”
One Line Summary: B cells present but dysfunctional, causing adult onset hypogammaglobulinemia with autoimmune and malignancy risk.
Defective Component: CD40 ligand (CD40L/CD154) on T helper cells in the most common X linked form. Without CD40L, B cells cannot undergo class switching from IgM to IgG, IgA, or IgE.
Pathophysiology: T helper cells normally express CD40L which binds CD40 on B cells to trigger isotype switching and germinal center formation. Without this signal, B cells are trapped producing only IgM. The result is elevated IgM with deficient IgG, IgA, and IgE. Additionally, the CD40L defect impairs macrophage activation, making patients vulnerable to opportunistic infections.
Genetic Mutation: CD40LG gene on Xq26.3 (X linked form). Autosomal recessive forms exist involving AID or UNG genes.
Inheritance: X linked recessive (most common form). Boys affected.
Clinical Presentation: Recurrent sinopulmonary infections, opportunistic infections including Pneumocystis jirovecii pneumonia and Cryptosporidium diarrhea (with risk of sclerosing cholangitis). Patients also develop neutropenia and oral ulcers.
Lab Findings: Markedly elevated IgM. Very low or absent IgG, IgA, and IgE. Normal B cell numbers.
NBME Buzzwords: Boy with elevated IgM, low IgG, Pneumocystis, Cryptosporidium, failure to class switch.
Common Exam Trap: Despite being classified as a B cell disorder on many charts, Hyper IgM is fundamentally a T cell signaling defect (CD40L is on T cells). NBME may test this by asking which cell is primarily defective, and the answer is the T helper cell.
One Line Summary: Defective CD40L on T cells prevents B cell class switching, causing elevated IgM with absent IgG/IgA/IgE and vulnerability to Pneumocystis and Cryptosporidium.
T cell deficiency disorders compromise cell mediated immunity and, because T helper cells are required for B cell class switching and activation, often secondarily impair humoral immunity. T cell deficiencies present early in life with viral infections, fungal infections, and opportunistic organisms like Pneumocystis jirovecii and Candida. The thymus plays a central role in T cell maturation, so any disorder affecting thymic development, such as thymic aplasia in DiGeorge syndrome, will produce T cell deficiency.
Defective Component: Failure of the third and fourth pharyngeal pouches to develop, resulting in thymic aplasia or hypoplasia, absent parathyroids, and cardiac outflow tract defects.
Pathophysiology: The 22q11.2 microdeletion disrupts TBX1 and other genes critical for pharyngeal pouch development. The thymus is the site of T cell maturation, so without a thymus, T cells cannot develop. The parathyroid glands also derive from the third pharyngeal pouch, causing hypoparathyroidism and hypocalcemia.
Genetic Mutation: Microdeletion at chromosome 22q11.2. Detected by FISH or chromosomal microarray.
Inheritance: Autosomal dominant, but most cases are de novo deletions.
Clinical Presentation: Neonatal tetany from hypocalcemia. Cardiac defects (tetralogy of Fallot, truncus arteriosus, interrupted aortic arch). Characteristic facial features (low set ears, short philtrum, micrognathia). Cleft palate. Recurrent viral and fungal infections. Variable severity of immunodeficiency depending on degree of thymic hypoplasia.
Lab Findings: Low T cell count on flow cytometry. Low PTH and calcium. Absent thymic shadow on chest X ray. Immunoglobulins may be normal or decreased depending on severity.
NBME Buzzwords: Newborn with tetany, cardiac defect, absent thymic shadow, hypocalcemia, 22q11.2 deletion.
22q11.2 Microdeletion
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Failed 3rd and 4th Pharyngeal Pouch Development
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Thymic Aplasia + Absent Parathyroids + Cardiac Defects
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No T Cell Maturation + Hypocalcemia + Conotruncal Anomalies
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Viral/Fungal Infections + Neonatal Tetany + Heart Failure
Common Exam Trap: The NBME may present a newborn with seizures (from hypocalcemia) and a cardiac murmur. The question might not mention the immune system at all, but the combination of tetany plus cardiac defect should immediately trigger DiGeorge. Do not get distracted by the seizure workup.
High Yield Mnemonic: “CATCH 22: Cardiac defects, Abnormal facies, Thymic aplasia, Cleft palate, Hypocalcemia, chromosome 22q11.2.”
One Line Summary: 22q11.2 deletion causes thymic aplasia (T cell deficit), absent parathyroids (hypocalcemia/tetany), and conotruncal cardiac defects. Use CATCH 22 to remember the full DiGeorge syndrome presentation on USMLE.
Defective Component: IL-12 receptor on T cells and NK cells. Without IL 12 signaling, TH1 differentiation fails and interferon gamma production is severely impaired.
Pathophysiology: The IL-12/IFN gamma axis is critical for macrophage activation and granuloma formation. When the IL-12 receptor is defective, macrophages cannot be activated to kill intracellular organisms, particularly Mycobacteria. This is why patients present with disseminated mycobacterial infections even from attenuated BCG vaccine strains.
Inheritance: Autosomal recessive.
Clinical Presentation: Disseminated mycobacterial infections (both typical and atypical Mycobacteria). Disseminated Salmonella. Poor granuloma formation. Often presents in infancy or early childhood after BCG vaccination in endemic areas.
NBME Buzzwords: Disseminated Mycobacterium, BCG vaccine complication, defective IFN gamma axis, poor granuloma formation.
One Line Summary: IL-12 receptor deficiency impairs the TH1/IFN gamma/macrophage axis causing susceptibility to Mycobacteria and Salmonella.
IL 12 Receptor Defect
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Failed TH1 Differentiation
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No IFN Gamma Production
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Macrophages Cannot Activate
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Disseminated Mycobacteria and Salmonella
Defective Component: T cell dysfunction specifically against Candida species. Often associated with defective TH17 responses (IL 17 pathway) or AIRE gene mutations.
Clinical Presentation: Persistent and recurrent Candida infections of the skin, nails, and mucous membranes. Patients do not develop systemic candidiasis in most cases. Associated with endocrinopathies (hypoparathyroidism, adrenal insufficiency) when linked to APECED/APS1 syndrome.
NBME Buzzwords: Chronic oral and nail candidiasis, endocrine dysfunction, T cell specific to Candida.
One Line Summary: Selective T cell defect against Candida causing chronic mucocutaneous (not systemic) candidal infections, often with endocrinopathy.
Combined immunodeficiency affects both T and B cell arms of adaptive immunity. These are the most severe primary immunodeficiency diseases and typically present in early infancy with failure to thrive, chronic diarrhea, and severe opportunistic infections. Without treatment, most combined immunodeficiencies are fatal within the first year of life. Understanding SCID genetics and the different variant patterns is one of the highest yield immunology topics on Step 1.
Defective Component: Multiple genetic defects can cause SCID. The most common is IL 2 receptor gamma chain (IL2RG) mutation (X linked SCID). Others include adenosine deaminase (ADA) deficiency (autosomal recessive) and RAG1/RAG2 mutations.
Pathophysiology: IL 2 receptor gamma chain is the common gamma chain shared by receptors for IL 2, IL 4, IL 7, IL 9, IL 15, and IL 21. Mutation eliminates signaling through all these cytokines, devastating T cell development and secondarily impairing B and NK cell function. ADA deficiency causes accumulation of toxic deoxyadenosine metabolites that kill lymphocyte precursors. RAG mutations prevent V(D)J recombination needed for T and B cell receptor diversity.
IL 2R Gamma Chain Mutation (X linked SCID)
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Loss of Signaling Through IL 2, IL 4, IL 7, IL 9, IL 15, IL 21
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Failed T Cell Development
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Secondarily Failed B and NK Cell Activation
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No Adaptive Immunity
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Fatal Opportunistic Infections Without Transplant
Inheritance: X linked recessive (IL2RG, most common). Autosomal recessive (ADA, RAG1/RAG2, JAK3).
Clinical Presentation: Presents within the first weeks to months of life. Failure to thrive, chronic diarrhea, oral thrush, Pneumocystis jirovecii pneumonia, disseminated CMV, chronic RSV, recurrent and severe skin infections. Absent thymic shadow on chest X ray. Graft versus host disease from nonirradiated blood products or transplacental maternal T cells.
Lab Findings: Absent or severely decreased T cells. B cells may be present but nonfunctional (T minus B plus NK minus in X linked SCID). ADA deficiency causes T minus B minus NK minus. Immunoglobulins are very low. Lymphopenia is profound.
Diagnostic Tests: Newborn screening with TREC (T cell receptor excision circles) assay. Flow cytometry for T, B, NK cell subsets. Genetic testing for specific mutations.
Pharmacology Correlation: Definitive treatment is hematopoietic stem cell transplant. ADA deficiency can be treated with PEG ADA enzyme replacement or gene therapy. Live vaccines are absolutely contraindicated.
NBME Buzzwords: Infant with failure to thrive, absent thymic shadow, Pneumocystis, chronic diarrhea, severe lymphopenia.
Common Exam Trap: NBME may describe an infant who develops a disseminated rash after receiving a live vaccine (like rotavirus or BCG). This scenario strongly points to SCID. Another trap is asking about GVHD in an infant who received nonirradiated blood.
High Yield Mnemonic: “SCID = Severe, Combined, Infections from Day one. No T, No B, No life without transplant.”
One Line Summary: The most severe primary immunodeficiency, caused by defects in IL2RG (most common), ADA, or RAG genes, presenting with early onset opportunistic infections and requiring stem cell transplant for survival.
Defective Component: WASP (Wiskott Aldrich Syndrome Protein) gene mutation. WASP is involved in actin cytoskeleton reorganization in hematopoietic cells, affecting T cells, B cells, and platelets.
Pathophysiology: Defective WASP impairs immune cell migration, signaling, and immune synapse formation. Platelets are small and dysfunctional. Progressive T cell dysfunction develops over time.
Inheritance: X linked recessive.
Clinical Presentation: Classic triad of eczema, thrombocytopenia (with small platelets), and recurrent infections. Increased risk of autoimmune disease and lymphoma.
Lab Findings: Thrombocytopenia with small platelets (low MPV). Low IgM, elevated IgA and IgE. Progressively declining T cell function.
NBME Buzzwords: Boy with eczema, thrombocytopenia, small platelets, recurrent infections.
High Yield Mnemonic: “WAS Has WET Problems: W = Wiskott Aldrich, E = Eczema, T = Thrombocytopenia.”
One Line Summary: WASP gene mutation causes the triad of eczema, thrombocytopenia with small platelets, and combined immunodeficiency in boys.
Defective Component: ATM (Ataxia Telangiectasia Mutated) gene. ATM is a kinase that detects DNA double strand breaks and activates repair pathways.
Pathophysiology: Without ATM, cells cannot repair DNA damage properly. This affects V(D)J recombination (impairing lymphocyte development), causes cerebellar degeneration, and leads to increased sensitivity to ionizing radiation and high cancer risk.
Inheritance: Autosomal recessive.
Clinical Presentation: Progressive cerebellar ataxia (starts around age 2 to 3), oculocutaneous telangiectasias (spider angiomas on face and conjunctivae), recurrent sinopulmonary infections, and markedly increased risk of lymphoma and leukemia.
Lab Findings: Low IgA (most consistent finding). Elevated alpha fetoprotein (AFP). Decreased T cells. Increased sensitivity to ionizing radiation on chromosomal breakage studies.
NBME Buzzwords: Child with ataxia, spider angiomas on the face, elevated AFP, low IgA, increased cancer risk.
Common Exam Trap: Elevated AFP in a child should not be confused with hepatocellular carcinoma (adult association). In a child with ataxia and telangiectasias, elevated AFP is diagnostic of Ataxia Telangiectasia.
One Line Summary: ATM gene mutation causes defective DNA repair leading to cerebellar ataxia, telangiectasias, IgA deficiency, elevated AFP, and increased lymphoma risk.
Defective Component: STAT3 mutation (autosomal dominant form, most common). STAT3 is a transcription factor involved in multiple cytokine signaling pathways.
Pathophysiology: Defective STAT3 impairs TH17 differentiation, reducing neutrophil recruitment to sites of infection. This creates “cold” (noninflamed) abscesses because the normal inflammatory response is blunted. Skeletal and dental abnormalities result from disrupted signaling in connective tissue.
Inheritance: Autosomal dominant (STAT3 mutation).
Clinical Presentation: Recurrent staphylococcal skin abscesses that are “cold” (lack warmth, erythema, and tenderness). Coarse facial features. Retained primary (baby) teeth. Pathologic fractures. Severe eczema. Eosinophilia.
Lab Findings: Massively elevated IgE (often greater than 2000 IU/mL). Eosinophilia. Normal other immunoglobulin levels.
NBME Buzzwords: Cold abscesses, coarse facies, retained primary teeth, pathologic fractures, very high IgE, eosinophilia.
High Yield Mnemonic: “FATED: Facies (coarse), Abscesses (cold), Teeth (retained), Eczema, elevated IgE (Dermatitis).”
One Line Summary: STAT3 mutation causes defective TH17 response leading to cold staphylococcal abscesses, coarse facies, retained teeth, and massively elevated IgE.
Phagocyte dysfunction diseases impair the ability of neutrophils and macrophages to kill engulfed organisms. Patients present with recurrent bacterial and fungal abscesses, poor wound healing, and granuloma formation. The key distinguishing feature from B cell and T cell deficiency disorders is that phagocyte disorders produce localized abscesses rather than disseminated opportunistic infections. Chronic granulomatous disease and leukocyte adhesion deficiency are the two most tested phagocyte disorders on USMLE.
Defective Component: NADPH oxidase complex. The most common mutation is in the CYBB gene encoding gp91phox (X linked). Autosomal recessive forms involve p47phox, p67phox, or p22phox subunits.
Pathophysiology: NADPH oxidase generates the superoxide radical (respiratory burst) that is essential for killing phagocytosed organisms. Without the respiratory burst, neutrophils and macrophages can engulf bacteria and fungi but cannot kill them. The body compensates by forming granulomas around the persistent infections. Catalase positive organisms are specifically dangerous because they destroy their own hydrogen peroxide, removing the only remaining bactericidal mechanism.
NADPH Oxidase Mutation
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No Superoxide Production (Respiratory Burst Fails)
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Phagocytes Engulf But Cannot Kill
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Persistent Intracellular Organisms
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Granuloma Formation + Recurrent Abscesses
Inheritance: X linked recessive (most common, CYBB gene). Autosomal recessive forms exist.
Most Common Infections: Catalase positive organisms specifically. Staphylococcus aureus, Aspergillus, Serratia marcescens, Nocardia, Burkholderia cepacia, Escherichia coli.
| UIT Teaching Pearl: Catalase Positive Organisms Mnemonic For CGD Use the mnemonic SPACE BANS to remember the catalase positive organisms that cause problems in CGD: S = Serratia marcescens P = Pseudomonas cepacia (Burkholderia) A = Aspergillus C = Candida (some species) E = E. coli B = Burkholderia cepacia A = Aspergillus (so high yield it bears repeating) N = Nocardia S = Staphylococcus aureus Why catalase matters: catalase positive organisms break down their own H2O2. In CGD, since the neutrophil cannot produce its own reactive oxygen species, the only residual killing mechanism is the small amount of H2O2 produced by the organism itself. Catalase positive organisms destroy even that, making them essentially unkillable by CGD neutrophils. |
Lab Findings: Negative nitroblue tetrazolium (NBT) test (no color change from yellow to blue). Abnormal dihydrorhodamine (DHR) flow cytometry (no fluorescence shift). Both tests measure NADPH oxidase function.
Pharmacology Correlation: Prophylactic trimethoprim sulfamethoxazole and itraconazole. Interferon gamma therapy to enhance residual phagocyte function. Stem cell transplant is curative.
NBME Buzzwords: Recurrent abscesses, granulomas, catalase positive organisms, negative NBT test, abnormal DHR assay.
One Line Summary: NADPH oxidase deficiency eliminates the respiratory burst, making neutrophils unable to kill catalase positive organisms, diagnosed by negative NBT or abnormal DHR.
Defective Component: CD18 (beta 2 integrin subunit). CD18 pairs with CD11a, CD11b, or CD11c to form LFA 1, Mac 1, and CR3, which are required for neutrophil adhesion to endothelium and migration into tissues.
Pathophysiology: Without functional integrins, neutrophils cannot adhere to blood vessel walls and cannot migrate to infection sites. This means infections occur but there is no pus formation despite very high circulating neutrophil counts. The neutrophils are stuck in the bloodstream.
Inheritance: Autosomal recessive.
Clinical Presentation: Delayed separation of the umbilical cord (beyond 30 days, normally separates by 2 weeks). Recurrent skin and mucosal infections without pus formation. Poor wound healing. Severe periodontitis.
Lab Findings: Markedly elevated WBC count (neutrophilia) even between infections. Absent CD18 on flow cytometry. Absence of pus at infection sites despite high peripheral neutrophil counts.
NBME Buzzwords: Delayed umbilical cord separation, absent pus despite infection, very high WBC, recurrent skin infections.
Common Exam Trap: NBME may describe a neonate with omphalitis and delayed cord separation with a very high WBC count. The student might be confused by the high WBC and think the immune system is working. The key is recognizing that despite high circulating neutrophils, there is no pus at the infection site, meaning neutrophils cannot reach tissues.
One Line Summary: CD18 integrin deficiency prevents neutrophil adhesion and migration, causing delayed cord separation, absent pus, and elevated WBC.
Defective Component: LYST gene (lysosomal trafficking regulator). Defective microtubule mediated lysosomal transport causes formation of giant granules in neutrophils and other cells.
Pathophysiology: Defective phagolysosome formation impairs intracellular killing. Giant granules fuse abnormally. Melanocyte dysfunction causes partial albinism. Neuronal dysfunction causes peripheral neuropathy.
Inheritance: Autosomal recessive.
Clinical Presentation: Partial oculocutaneous albinism (light skin and silvery hair). Recurrent pyogenic infections. Peripheral neuropathy. Progressive neurodegeneration. Risk of accelerated phase (hemophagocytic lymphohistiocytosis).
Lab Findings: Giant azurophilic granules in neutrophils and other WBCs on peripheral blood smear. This is the pathognomonic finding.
NBME Buzzwords: Partial albinism, giant granules in neutrophils, peripheral neuropathy, silvery hair.
One Line Summary: LYST gene mutation causes defective lysosomal trafficking with giant granules, partial albinism, neuropathy, and recurrent pyogenic infections.
Defective Component: Myeloperoxidase enzyme in neutrophil azurophilic granules. MPO catalyzes the formation of hypochlorous acid (HOCl) from hydrogen peroxide and chloride, which is a potent microbicidal agent.
Pathophysiology: Without MPO, the HOCl dependent killing pathway is lost. However, other oxygen dependent killing mechanisms (superoxide, hydrogen peroxide) remain intact because NADPH oxidase is normal. This is why MPO deficiency is usually clinically mild.
Clinical Presentation: Most patients are asymptomatic. Increased risk of Candida infections in patients who also have diabetes mellitus.
Lab Findings: NBT test is normal (NADPH oxidase works). DHR is normal. Absent MPO staining on flow cytometry.
NBME Buzzwords: Most common inherited phagocyte deficiency. Clinically mild. Increased Candida risk only in diabetics. Normal NBT test.
One Line Summary: Most common inherited phagocyte defect, usually asymptomatic because NADPH oxidase remains functional, with Candida risk only in diabetic patients.
The complement system bridges innate and adaptive immunity. Complement deficiency disorders are classified by which components are missing and produce distinct clinical presentations. Early complement defects (C1 to C4) predispose to autoimmune disease, especially SLE. Terminal complement defects (C5 to C9) specifically predispose to recurrent Neisseria infections. Regulatory protein deficiencies produce uncontrolled complement activation with tissue damage. These complement deficiency disorders are high yield immunology topics for Step 1 and appear on nearly every NBME form.
Defective Component: Any component of the membrane attack complex (C5, C6, C7, C8, or C9).
Pathophysiology: The MAC (C5b through C9) forms pores in bacterial cell membranes, directly lysing gram negative organisms. Without MAC, patients cannot kill Neisseria species, which are uniquely susceptible to complement mediated lysis.
Inheritance: Autosomal recessive.
Clinical Presentation: Recurrent Neisseria meningitidis and Neisseria gonorrhoeae infections, typically presenting in adolescence or young adulthood. Patients may have repeated episodes of meningococcal meningitis.
Lab Findings: Low or absent CH50 (total hemolytic complement). Individual complement component levels identify the specific deficiency.
NBME Buzzwords: Teenager with recurrent meningococcal meningitis, low CH50.
Pharmacology Correlation: Meningococcal vaccination (especially with quadrivalent and serogroup B vaccines). Eculizumab (anti C5 monoclonal antibody) used in PNH also increases Neisseria risk.
One Line Summary: MAC deficiency specifically predisposes to recurrent Neisseria infections, diagnosed by low CH50.
Defective Component: C1 esterase inhibitor (C1INH), a serine protease inhibitor that regulates C1, kallikrein, and Factor XIIa.
Pathophysiology: Without C1INH, uncontrolled activation of kallikrein produces excess bradykinin. Bradykinin increases vascular permeability, causing episodic nonpitting angioedema of the face, larynx, and intestines. This is not histamine mediated, so antihistamines and epinephrine are ineffective.
Inheritance: Autosomal dominant.
Clinical Presentation: Recurrent episodes of painless, nonpitting angioedema affecting the face, extremities, larynx (risk of airway obstruction), and GI tract (abdominal pain, mimicking surgical abdomen). Not associated with urticaria or pruritus. Attacks often triggered by stress, trauma, or dental procedures.
Lab Findings: Low C4 (best screening test, low even between attacks). Low C1INH level or function. C3 is normal (distinguishes it from other complement disorders).
NBME Buzzwords: Recurrent angioedema without urticaria, unresponsive to epinephrine, low C4, positive family history.
Pharmacology Correlation: Acute attacks treated with C1INH concentrate, icatibant (bradykinin B2 receptor antagonist), or ecallantide (kallikrein inhibitor). Androgens (danazol) were historically used for prophylaxis but have been replaced by C1INH replacement and lanadelumab (anti kallikrein antibody). ACE inhibitors are contraindicated because they block bradykinin degradation.
Common Exam Trap: NBME may present a patient who develops angioedema after starting an ACE inhibitor. While this can happen in anyone taking ACE inhibitors, a patient with hereditary angioedema will have dramatically worsened symptoms. The mechanism is bradykinin accumulation, not allergy.
One Line Summary: C1INH deficiency causes excess bradykinin leading to recurrent angioedema without urticaria, diagnosed by low C4, and is not responsive to antihistamines or epinephrine.
Defective Component: GPI anchor synthesis due to somatic PIGA gene mutation in hematopoietic stem cells. Loss of GPI anchored proteins CD55 (DAF) and CD59 (MIRL) leaves red blood cells vulnerable to complement mediated lysis.
Pathophysiology: CD55 accelerates decay of C3 convertase. CD59 blocks MAC assembly on host cells. Without both, complement freely attacks the patient’s own red cells, causing intravascular hemolysis, particularly at night when slight respiratory acidosis lowers blood pH and activates complement.
Inheritance: Acquired somatic mutation (not inherited). The PIGA mutation occurs in a hematopoietic stem cell and expands clonally.
Clinical Presentation: Hemolytic anemia with dark morning urine (hemoglobinuria). Pancytopenia from associated bone marrow failure. Venous thrombosis (especially hepatic, cerebral, and abdominal veins), which is the leading cause of death. Strong association with aplastic anemia.
Lab Findings: Flow cytometry showing absence of CD55 and CD59 on red cells and WBCs (gold standard). Elevated LDH, low haptoglobin, positive urine hemosiderin. Negative direct Coombs (not autoimmune). Historically diagnosed with acidified serum (Ham) test.
NBME Buzzwords: Dark morning urine, pancytopenia, venous thrombosis (Budd Chiari), aplastic anemia association, absent CD55/CD59.
Pharmacology Correlation: Eculizumab (anti C5 monoclonal antibody) prevents MAC formation and reduces hemolysis. Patients on eculizumab must be vaccinated against Neisseria meningitidis. Hematopoietic stem cell transplant is curative.
One Line Summary: Acquired PIGA mutation causes loss of GPI anchored complement regulators (CD55, CD59), leading to complement mediated hemolysis, pancytopenia, and venous thrombosis.
| Feature | SCID | DiGeorge |
| Defect | Multiple gene defects (IL2RG, ADA, RAG) | 22q11.2 deletion |
| T cells | Absent | Low (variable) |
| B cells | Present but nonfunctional (X linked) or absent (ADA) | Usually normal |
| Thymus | Absent | Absent or hypoplastic |
| Calcium | Normal | Low (hypoparathyroidism) |
| Cardiac | No | Yes (conotruncal defects) |
| Key clue | Failure to thrive, Pneumocystis | Tetany, cardiac defect, facial anomalies |
| Feature | CGD | LAD Type 1 |
| Defect | NADPH oxidase (respiratory burst) | CD18 integrin (adhesion) |
| Neutrophils at site | Present but cannot kill | Cannot reach tissue |
| Pus | Yes (granulomas and abscesses) | No (absent despite infection) |
| WBC count | Normal to mildly elevated | Markedly elevated (stuck in blood) |
| Key organisms | Catalase positive (Staph, Aspergillus) | Broad bacterial infections |
| Diagnostic test | Negative NBT / abnormal DHR | Absent CD18 on flow cytometry |
| Unique clue | Granuloma formation | Delayed cord separation |
| Feature | Bruton (XLA) | CVID |
| Age of onset | After 6 months | Second to third decade |
| B cells | Absent | Present but dysfunctional |
| Immunoglobulins | All absent | All low |
| Sex | Males only (X linked) | Males and females |
| Autoimmune risk | Low | High |
| Lymphoma risk | Low | Elevated |
| Key clue | Boy, absent tonsils, no B cells | Adult, normal B cell count, autoimmunity |
| Feature | Hyper IgM | Hyper IgE (Job) |
| Defect | CD40L on T cells | STAT3 transcription factor |
| Elevated Ig | IgM | IgE |
| Key infections | Pneumocystis, Cryptosporidium | Staphylococcal cold abscesses |
| Inheritance | X linked recessive | Autosomal dominant |
| Unique features | Neutropenia, oral ulcers | Coarse facies, retained teeth, fractures |
| Mechanism | Failed class switching | Failed TH17 response |
| Feature | Chediak Higashi | MPO Deficiency |
| Defect | LYST gene (microtubule trafficking) | Myeloperoxidase enzyme |
| Clinical severity | Severe | Usually asymptomatic |
| Unique features | Partial albinism, neuropathy, giant granules | None (most patients healthy) |
| NBT test | Normal (NADPH oxidase intact) | Normal (NADPH oxidase intact) |
| Key risk | HLH (accelerated phase) | Candida only if diabetic |
This table maps opportunistic infections to their most likely immune defect association. In any USMLE immunology question, identifying the organism in the vignette is often the fastest path to identifying the underlying immunodeficiency. This is one of the most testable NBME immunology concepts and appears in multiple forms across every practice exam.
| Organism/Infection | Primary Immune Defect Association |
| Pneumocystis jirovecii | T cell defect (SCID, Hyper IgM, DiGeorge, HIV) |
| Candida (mucocutaneous) | T cell defect (Chronic mucocutaneous candidiasis, DiGeorge) |
| Candida (systemic, in diabetic) | MPO deficiency |
| Aspergillus | CGD (phagocyte defect), also neutropenia |
| Staphylococcus aureus abscesses | CGD, Hyper IgE syndrome |
| Serratia marcescens | CGD (catalase positive) |
| Neisseria meningitidis / gonorrhoeae | Terminal complement (C5 to C9) deficiency |
| Encapsulated bacteria (Pneumococcus, H. influenzae) | B cell defects (Bruton, CVID), complement (C3) |
| Giardia lamblia | IgA deficiency, CVID, Bruton |
| Cryptosporidium | Hyper IgM syndrome (sclerosing cholangitis risk) |
| Mycobacteria (disseminated) | IL 12 receptor deficiency, IFN gamma receptor deficiency |
| CMV / EBV / HSV (disseminated) | T cell / combined defects (SCID, DiGeorge) |
| Nocardia | CGD |
| Pseudomonas / Burkholderia cepacia | CGD |
This axis is one of the most testable integrated pathways on USMLE Step 1. Understanding this immunology flowchart connects macrophage function, T cell differentiation, and mycobacterial defense into a single coherent system that NBME tests repeatedly.
Macrophage Engulfs Mycobacterium
↓
Macrophage Secretes IL 12
↓
IL 12 Activates TH1 Cells and NK Cells
↓
TH1 Cells Produce IFN Gamma
↓
IFN Gamma Activates Macrophage Killing (Respiratory Burst Enhanced)
↓
Macrophage Kills Intracellular Mycobacteria
Any break in this loop causes susceptibility to Mycobacteria and Salmonella. This includes IL 12 deficiency, IL-12 receptor deficiency, IFN gamma deficiency, and IFN gamma receptor deficiency. The clinical presentation is identical: disseminated mycobacterial disease, often triggered by BCG vaccination.
| UIT Teaching Pearl • When you see disseminated Mycobacteria in a question stem, especially after BCG vaccination, do not think of CGD or SCID first. • Think IL 12/IFN gamma axis defect. The NBME specifically tests this pathway because it links innate and adaptive immunity beautifully. • The mnemonic: “BCG Gone Bad = IL 12/IFN Gamma Axis Broken.” |
The thymus is the central organ of T cell development. T cell precursors migrate from the bone marrow to the thymus where they undergo positive selection (in the cortex, can the T cell recognize self MHC?) and negative selection (in the medulla, does the T cell react too strongly to self antigens?). AIRE (autoimmune regulator) in the medulla drives expression of tissue specific antigens for negative selection.
Any defect in thymic development (DiGeorge) or thymic function produces T cell deficiency. Understanding the embryology is critical because the NBME specifically tests the pharyngeal pouch origin of the thymus (third pouch) and parathyroids (third and fourth pouches). This is why DiGeorge presents with both immune deficiency and hypoparathyroidism.10 High Yield USMLE Style MCQs: Immunodeficiency Disorders
A 7 month old boy is brought to the emergency department for his third episode of bacterial pneumonia. Physical examination reveals absent tonsils and no palpable lymph nodes. Laboratory studies show undetectable IgG, IgA, and IgM. Flow cytometry reveals absent CD19+ cells and normal CD3+ cells. Which of the following is the most likely diagnosis?
A. Common variable immunodeficiency
B. Bruton agammaglobulinemia
C. Selective IgA deficiency
D. Hyper IgM syndrome
E. Severe combined immunodeficiency
Correct Answer: B. Bruton agammaglobulinemia
Explanation: The combination of a boy (X linked), onset after 6 months (maternal IgG waned), absent tonsils and lymph nodes (no germinal centers), panhypogammaglobulinemia, and absent B cells (CD19+) with normal T cells (CD3+) is classic for Bruton agammaglobulinemia caused by BTK mutation. CVID has normal B cell numbers. IgA deficiency only affects IgA. Hyper IgM has elevated IgM. SCID would show absent T cells.
A newborn presents with seizures on day 2 of life. Serum calcium is 5.2 mg/dL. Chest X ray shows an absent thymic shadow and a right sided aortic arch. Which of the following is the most likely underlying genetic abnormality?
A. FMR1 trinucleotide expansion
B. Chromosome 22q11.2 microdeletion
C. Chromosome 15q deletion (maternal)
D. IL2RG mutation
E. CFTR mutation
Correct Answer: B. Chromosome 22q11.2 microdeletion
Explanation: Neonatal hypocalcemia (seizures), absent thymic shadow, and cardiac defect (right sided aortic arch is a conotruncal anomaly) form the triad of DiGeorge syndrome caused by 22q11.2 deletion. This disrupts third and fourth pharyngeal pouch development affecting the thymus and parathyroids.
A 2 year old boy has a history of recurrent liver and perianal abscesses. Cultures from the most recent abscess grow Aspergillus fumigatus. The dihydrorhodamine (DHR) test shows no fluorescence shift. Which enzyme is most likely deficient?
A. Myeloperoxidase
B. Adenosine deaminase
C. NADPH oxidase
D. Glucose 6 phosphate dehydrogenase
E. Sphingomyelinase
Correct Answer: C. NADPH oxidase
Explanation: Recurrent abscesses with catalase positive organisms (Aspergillus) and an abnormal DHR test (no fluorescence = no respiratory burst) is diagnostic of chronic granulomatous disease caused by NADPH oxidase deficiency. MPO deficiency would have a normal DHR. ADA deficiency causes SCID, not abscesses.
A 10 day old neonate is evaluated for omphalitis. The umbilical cord has not yet separated. Physical examination shows erythema around the umbilical stump but no purulent discharge. CBC shows a WBC count of 45,000/microL with 90% neutrophils. Which adhesion molecule is most likely deficient?
A. Selectin
B. CD18
C. ICAM 1
D. CD40 ligand
E. LFA 3
Correct Answer: B. CD18
Explanation: Delayed umbilical cord separation, omphalitis without pus formation, and markedly elevated WBC (neutrophils stuck in the bloodstream) is classic leukocyte adhesion deficiency type 1 caused by CD18 (beta 2 integrin) deficiency.
A 4 month old infant is admitted with Pneumocystis jirovecii pneumonia, oral thrush, and failure to thrive. Chest X ray shows an absent thymic shadow. Flow cytometry shows absent T cells, present but nonfunctional B cells, and absent NK cells. Which of the following genes is most likely mutated?
A. BTK
B. WASP
C. IL2RG
D. ATM
E. CYBB
Correct Answer: C. IL2RG
Explanation: SCID presenting with Pneumocystis, oral thrush, failure to thrive, absent thymic shadow, and T minus B plus NK minus pattern on flow cytometry points to IL2RG (common gamma chain) mutation, the most common cause of X linked SCID. The gamma chain is shared by IL 2, IL 4, IL 7, IL 9, IL 15, and IL 21 receptors.
A 15 year old boy presents with his second episode of meningococcal meningitis. His older brother also had meningococcal meningitis at age 13. Serum CH50 is undetectable. Which complement components are most likely deficient?
A. C1 and C2
B. C3
C. C1 esterase inhibitor
D. C5 through C9
E. Factor B and Factor D
Correct Answer: D. C5 through C9
Explanation: Recurrent Neisseria meningitidis infections in a teenager with family history and absent CH50 is terminal complement (MAC) deficiency. C5 through C9 form the membrane attack complex needed to lyse Neisseria. Early complement deficiency (C1 to C4) causes SLE. C1INH deficiency causes hereditary angioedema.
A 3 year old boy presents with eczema, recurrent otitis media, and easy bruising. Peripheral blood smear shows small platelets. Laboratory studies show low IgM, elevated IgA, and elevated IgE. Which of the following proteins is most likely deficient?
A. STAT3
B. BTK
C. WASP
D. ATM
E. LYST
Correct Answer: C. WASP
Explanation: The triad of eczema, thrombocytopenia with small platelets, and recurrent infections in a boy (X linked) with low IgM and elevated IgA/IgE is Wiskott Aldrich syndrome caused by WASP gene mutation. Job syndrome (STAT3) has elevated IgE but coarse facies and cold abscesses without thrombocytopenia.
A 5 year old girl is evaluated for progressive gait instability. Physical examination reveals telangiectasias on the conjunctivae and face. Laboratory studies show elevated alpha fetoprotein and low IgA. Which of the following is the most likely underlying defect?
A. Defective DNA double strand break repair
B. Defective NADPH oxidase
C. Defective lysosomal trafficking
D. Defective class switching
E. Defective thymic development
Correct Answer: A. Defective DNA double strand break repair
Explanation: Cerebellar ataxia (gait instability), oculocutaneous telangiectasias, elevated AFP, and low IgA is ataxia telangiectasia caused by ATM gene mutation. ATM is a kinase that detects and repairs DNA double strand breaks. Its loss impairs V(D)J recombination and causes increased radiation sensitivity and cancer risk.
A 1 year old boy presents with recurrent staphylococcal skin abscesses that are remarkably nontender and lack surrounding erythema. Physical examination shows coarse facial features and eczematoid rash. Serum IgE is 5,400 IU/mL. Which transcription factor is most likely mutated?
A. STAT1
B. STAT3
C. STAT5
D. FOXP3
E. T bet
Correct Answer: B. STAT3
Explanation: Cold (noninflamed) staphylococcal abscesses, coarse facies, eczema, and massively elevated IgE is Hyper IgE syndrome (Job syndrome) caused by STAT3 mutation. STAT3 is critical for TH17 differentiation, and without TH17 cells, neutrophil recruitment to infection sites is impaired, producing the characteristic cold abscesses.
A 28 year old woman presents to the emergency department with severe facial and lip swelling without urticaria. She reports multiple similar episodes over the past 5 years. Her mother has similar symptoms. Epinephrine provides no relief. Serum C4 is low. C3 is normal. Which of the following mediators is most directly responsible for her symptoms?
A. Histamine
B. Bradykinin
C. Leukotriene C4
D. Prostaglandin D2
E. Serotonin
Correct Answer: B. Bradykinin
Explanation: Recurrent angioedema without urticaria, unresponsive to epinephrine, with family history and low C4 with normal C3 is hereditary angioedema from C1 esterase inhibitor deficiency. The swelling is mediated by excess bradykinin (not histamine), which is why antihistamines and epinephrine fail. Bradykinin increases vascular permeability directly.
• Bruton = X linked, absent B cells, panhypogammaglobulinemia, BTK mutation, boy after 6 months.
• Selective IgA deficiency = most common primary immunodeficiency, anaphylaxis to blood transfusion, Giardia.
• CVID = adult onset, B cells present but dysfunctional, autoimmune risk, lymphoma risk.
• Hyper IgM = CD40L defect on T cells, elevated IgM, no class switching, Pneumocystis, Cryptosporidium.
• DiGeorge = 22q11.2 deletion, CATCH 22 (Cardiac, Abnormal facies, Thymic aplasia, Cleft palate, Hypocalcemia, 22), absent thymic shadow.
• SCID = most severe, multiple gene causes (IL2RG most common), absent thymic shadow, fatal without transplant.
• Wiskott Aldrich = WET (Wiskott, Eczema, Thrombocytopenia with small platelets), WASP gene, X linked.
• Ataxia telangiectasia = ATM gene, cerebellar ataxia, telangiectasias, elevated AFP, low IgA, cancer risk.
• Hyper IgE (Job) = STAT3, cold abscesses, coarse facies, retained teeth, IgE greater than 2000.
• CGD = NADPH oxidase, catalase positive organisms (SPACE BANS), negative NBT, abnormal DHR, granulomas.
• LAD = CD18, delayed cord separation, no pus, elevated WBC.
• Chediak Higashi = LYST gene, giant granules, partial albinism, neuropathy.
• MPO deficiency = most common phagocyte defect, usually asymptomatic, Candida only if diabetic.
• C5 to C9 deficiency = recurrent Neisseria, low CH50, vaccinate against meningococcus.
• C1INH deficiency = hereditary angioedema, bradykinin mediated, low C4, normal C3, ACE inhibitors contraindicated.
• PNH = acquired PIGA mutation, loss of CD55/CD59, hemolysis, pancytopenia, venous thrombosis, treated with eculizumab.
• IL 12/IFN gamma axis defect = disseminated Mycobacteria, BCG complications.
• Catalase positive organisms in CGD: Staphylococcus, Pseudomonas/Burkholderia, Aspergillus, Serratia, Nocardia, E. coli.
• B cell defects present after 6 months (maternal IgG wanes). T cell defects present earlier.
• Encapsulated bacteria = B cell or complement. Viral/fungal = T cell. Catalase positive = CGD. Neisseria = terminal complement.
SCID is caused by genetic defects (IL2RG, ADA, RAG) that prevent T cell development from the stem cell level, affecting both T and B cell function and requiring stem cell transplant. DiGeorge is caused by 22q11.2 deletion affecting thymic development specifically, with associated hypoparathyroidism and cardiac defects. SCID is more severe and presents earlier with broader opportunistic infections.
In CGD, neutrophils lack NADPH oxidase and cannot produce their own reactive oxygen species. The only residual killing mechanism comes from hydrogen peroxide produced by the bacteria themselves. Catalase positive organisms destroy their own H2O2, eliminating this last defense mechanism. Catalase negative organisms (like Streptococcus) still produce H2O2 that can contribute to their own killing even in CGD neutrophils.
Terminal complement deficiency (C5 through C9). The membrane attack complex is specifically required to lyse Neisseria species. Patients with any MAC component deficiency present with recurrent meningococcal and gonococcal infections, typically in adolescence. The diagnostic test is CH50 which will be low or absent.
SCID, DiGeorge syndrome, and CGD are consistently the most tested immunodeficiencies on USMLE Step 1. However, selective IgA deficiency is the most commonly tested in the context of transfusion reactions, and terminal complement deficiency is most tested in the context of recurrent Neisseria. The NBME tests pattern recognition across all categories rather than depth in any single disorder.
The most effective approach is to categorize by immune component (B cell, T cell, combined, phagocyte, complement), then learn the signature clue for each disorder. Use the organism type to narrow the category, use the age and sex to narrow further, and use the lab finding to confirm. Mnemonics like WET for Wiskott Aldrich, SPACE BANS for CGD organisms, and FATED for Job syndrome anchor the details.
Both cause low immunoglobulin levels, but in Bruton agammaglobulinemia B cells are completely absent on flow cytometry due to BTK gene mutation, presenting in boys after 6 months. In CVID, B cells are present in normal numbers but fail to differentiate into plasma cells, presenting in adults of either sex in their 20s or 30s with additional autoimmune disease and lymphoma risk.
A negative nitroblue tetrazolium (NBT) test means NADPH oxidase is not functioning in neutrophils. Normally, the respiratory burst produces superoxide that reduces yellow NBT dye to blue formazan. No color change indicates absent respiratory burst, which is diagnostic of chronic granulomatous disease. The dihydrorhodamine (DHR) flow cytometry test has largely replaced NBT as the preferred diagnostic test for CGD.
Normal umbilical cord separation requires neutrophils to migrate into the devitalized stump tissue and degrade it through inflammation. In leukocyte adhesion deficiency type 1, neutrophils lack CD18 (beta 2 integrin) and cannot adhere to blood vessel walls or migrate into tissues. Without neutrophil migration to the cord stump, the degradation process fails and the cord remains attached beyond 30 days.
Immunology becomes much easier when you stop treating every disorder as a separate memorization task. At IMG Helping Hands, we emphasize recognizing recurring NBME-style clinical patterns so you can move from recall-based studying to reasoning-based answers. Most Step 1 immunology questions can be solved faster when you connect immune pathways with infections, lab findings, and patient presentation instead of memorizing isolated charts.
IMG HELPING HANDS – UIT IMMUNOLOGY SYSTEM
The UIT program at IMG Helping Hands teaches immunodeficiency disorders through mechanism-based flowcharts, infection pattern recognition, and NBME-style clinical reasoning designed specifically for IMGs.
Learn how to identify immune defects from vignette clues like organism type, age of onset, and lab findings, quickly and confidently on exam day.
Pattern recognition. Clinical reasoning. Built for Step 1.
• First Aid for the USMLE Step 1, 2026 Edition. Immunology and Immunodeficiency chapters.
• Pathoma: Fundamentals of Pathology. Immunology section by Dr. Husain Sattar.
• Robbins and Cotran Pathologic Basis of Disease, 10th Edition. Diseases of the Immune System.
• Janeway’s Immunobiology, 10th Edition. Primary Immunodeficiency chapters.
• NBME Subject Examination and USMLE Step 1 Content Outline, 2025 to 2026.
• UWorld Step 1 QBank. Immunology and Immunodeficiency explanations.
Articles published by IMG Helping Hands are prepared by our team using information from direct experience, publicly available resources, and educational references. AI tools may be used to assist with drafting, proofreading, and formatting; however, all content undergoes review and approval before publication.
The information provided is intended for educational purposes only. Requirements, policies, and processes may change over time. Readers should consult official sources for the most current information.
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