USMLE Step 1 & 2 Hypersensitivity and Immunodeficiency

Last updated: May 2, 2026

Hypersensitivity and Immunodeficiency questions are one of the highest-leverage areas to study for the USMLE Step 1 & 2. This guide breaks down the rule, the elements you need to recognize, the named traps that catch most students, and a memory aid that scales to test day. Read it once, then practice the same sub-topic adaptively in the app.

The rule

Hypersensitivity reactions are categorized into four mechanistic types (I-IV) based on the immune effector causing tissue injury: IgE-mast cell (I), antibody-mediated cytotoxic (II), immune complex (III), and T-cell mediated (IV). Primary immunodeficiencies are classified by which arm of immunity is broken — B-cell (recurrent encapsulated bacteria, sinopulmonary), T-cell (viral, fungal, opportunistic intracellular), combined (everything early in life), phagocyte (catalase-positive bacteria, abscesses), or complement (Neisseria, lupus-like). On exam day, your job is to read three lines of clinical or laboratory pattern and reach for the right bucket — the diagnosis itself is usually the close-mimic distractor.

Elements breakdown

Type I (Immediate, IgE-mediated)

Allergen crosslinks IgE bound to FcεRI on mast cells/basophils, causing immediate degranulation and a late phase 4-12 hours later.

  • Onset minutes after exposure
  • Mast cell tryptase elevated
  • Histamine, leukotrienes, prostaglandins released
  • Th2/IL-4 class switching to IgE

Common examples:

  • Anaphylaxis
  • Atopy, allergic rhinitis
  • Acute urticaria
  • Allergic asthma

Type II (Antibody-mediated cytotoxic)

IgG or IgM binds cell-surface or matrix antigen, triggering complement, opsonization, ADCC, or receptor dysfunction.

  • Direct/indirect Coombs positive in cytopenias
  • Linear immunofluorescence on biopsy
  • Auto-antibody to specific receptor or matrix
  • Hours to days onset

Common examples:

  • Autoimmune hemolytic anemia
  • Goodpasture (anti-GBM)
  • Myasthenia gravis (anti-AChR)
  • Acute hemolytic transfusion reaction

Type III (Immune complex)

Soluble antigen-antibody complexes deposit in vessels, joints, glomeruli; activate complement and recruit neutrophils.

  • Low serum C3/C4
  • Granular ('lumpy-bumpy') immunofluorescence
  • Onset 1-3 weeks after antigen
  • Vasculitis, arthralgias, nephritis triad

Common examples:

  • Serum sickness
  • SLE
  • Post-streptococcal GN
  • Polyarteritis nodosa

Type IV (Delayed, T-cell mediated)

Sensitized CD4+ Th1 cells (or CD8+ CTLs) recognize antigen and release IFN-γ, recruiting macrophages 48-72 hours later.

  • No antibody, no complement
  • Onset 2-4 days after exposure
  • Granulomas with chronic exposure
  • Mononuclear infiltrate on biopsy

Common examples:

  • PPD/TB skin test
  • Contact dermatitis (poison ivy, nickel)
  • Type 1 diabetes
  • Multiple sclerosis
  • Graft rejection (acute cellular)

B-cell deficiency

Defective antibody production; presents after 6 months as maternal IgG wanes.

  • Recurrent encapsulated bacteria (S. pneumo, H. flu, N. meningitidis)
  • Sinopulmonary, otitis, bacteremia
  • Low immunoglobulins on quantitative panel
  • Giardia and enterovirus susceptibility

Common examples:

  • X-linked (Bruton) agammaglobulinemia — BTK mutation
  • Common variable immunodeficiency (CVID)
  • Selective IgA deficiency (anaphylaxis to IVIG)

T-cell / combined deficiency

Defective cellular immunity, often with secondary B-cell dysfunction.

  • Severe viral, fungal, PJP, mycobacterial infections
  • Failure to thrive in infancy
  • Chronic diarrhea, thrush
  • Absent thymic shadow on CXR

Common examples:

  • SCID (IL-2Rγ, ADA)
  • DiGeorge (22q11.2, no thymus, hypocalcemia, conotruncal defects)
  • Wiskott-Aldrich (WATER: Wiskott, Atopy, Thrombocytopenia, Eczema, Recurrent infections)
  • Hyper-IgM (CD40L)

Phagocyte defects

Impaired killing or migration by neutrophils/macrophages.

  • Skin and visceral abscesses
  • Catalase-positive organisms (S. aureus, Aspergillus, Serratia, Nocardia, Burkholderia)
  • Delayed umbilical cord separation in LAD
  • Abnormal NBT/dihydrorhodamine in CGD

Common examples:

  • Chronic granulomatous disease (NADPH oxidase)
  • Leukocyte adhesion deficiency type 1 (CD18)
  • Chediak-Higashi (LYST, giant granules)
  • Hyper-IgE / Job (STAT3, FATED: coarse Facies, cold Abscesses, retained Teeth, hyper-IgE, Dermatologic)

Complement deficiency

Loss of complement components or regulators causes specific infection or autoimmunity patterns.

  • C5-C9 (MAC) → recurrent Neisseria
  • C1-C4 → SLE-like syndrome
  • C1 esterase inhibitor → hereditary angioedema
  • DAF/CD55 → paroxysmal nocturnal hemoglobinuria

Common examples:

  • Recurrent meningococcemia
  • SLE in early-component deficiency
  • Hereditary angioedema (no urticaria, ACE-inhibitor-like swelling)

Common patterns and traps

The Bug-to-Compartment Map

Recurrent infections with a specific microbial signature point reflexively to one immune compartment. Encapsulated bacteria (S. pneumo, H. flu, N. meningitidis) — humoral/B-cell. PJP, candida, CMV, mycobacteria — T-cell. S. aureus and Aspergillus abscesses — neutrophil/phagocyte. Recurrent Neisseria — terminal complement.

A vignette names the organism in the first or second sentence; the correct answer is the immunodeficiency whose compartment normally clears that organism.

The Timing Discriminator for Hypersensitivity

The interval between exposure and reaction is diagnostic. Minutes — Type I. Hours to a day — Type II (cytotoxic, e.g., transfusion reaction). 7-21 days after antigen — Type III (serum sickness window). 48-72 hours — Type IV. Stems often hand you the time interval as a 'gift' that decides the type.

A patient develops urticaria, fever, and arthralgias '10 days after starting' a new drug — Type III serum sickness, not Type I drug allergy.

The Linear-vs-Granular Immunofluorescence Trap

Renal biopsy IF patterns separate Type II from Type III. Linear (smooth ribbon) IgG = anti-GBM (Goodpasture). Granular ('lumpy-bumpy') IgG/C3 = immune complex (post-strep GN, lupus, IgA nephropathy in mesangium). Candidates who don't memorize this distinction lose Type II vs III questions on biopsy stems.

A vignette describes hemoptysis, hematuria, and 'linear deposition of IgG along the glomerular basement membrane'; the answer is anti-GBM (Type II), not immune-complex GN (Type III).

The Age-of-Onset Sieve

Onset before 6 months suggests T-cell or combined defect (SCID, DiGeorge) because maternal IgG is still protecting humoral immunity. Onset 6-12 months suggests B-cell defect (XLA). Onset in adolescence/adulthood with similar B-cell-pattern infections suggests CVID.

A 3-month-old with thrush, PJP pneumonia, and lymphopenia — SCID. A 9-month-old boy with H. flu otitis and pneumococcal pneumonia — XLA. A 28-year-old with recurrent sinopulmonary infections and low IgG — CVID.

The Auto-Antibody-to-Receptor Map

Type II reactions where the antibody targets a receptor produce signature clinical pictures: anti-AChR → myasthenia gravis (fatigable weakness), anti-TSH receptor (stimulating) → Graves, anti-desmoglein → pemphigus vulgaris, anti-hemidesmosome → bullous pemphigoid. The mechanism is the answer when the stem describes the auto-antibody.

A woman has fatigable ptosis worse at end of day; serology shows antibodies against the post-synaptic acetylcholine receptor — Type II hypersensitivity by receptor blockade.

How it works

Imagine a 4-year-old boy with three episodes of pneumococcal pneumonia and one bout of H. influenzae meningitis since age 8 months. Quantitative immunoglobulins show IgG, IgA, and IgM all undetectable; flow cytometry shows essentially no circulating CD19+ B cells. The pattern — recurrent encapsulated bacteria, onset after maternal IgG wanes around 6 months, absent B cells — points to X-linked (Bruton) agammaglobulinemia from a BTK mutation that blocks pre-B to immature-B transition. The trap is to pick CVID, which presents later (typically 20s-30s) and has normal B-cell counts with low immunoglobulin output. To work hypersensitivity and immunodeficiency questions reliably, anchor on two pieces of data: timing (minutes vs hours vs days vs weeks) and the cell or molecule doing the damage. If you can name those, the bucket is forced and the answer choices sort themselves.

Worked examples

Worked Example 1

Which of the following best explains this patient's underlying defect?

  • A Defective NADPH oxidase impairing the respiratory burst in neutrophils
  • B Mutation in Bruton tyrosine kinase blocking pre-B to immature-B cell development ✓ Correct
  • C Failure of thymic development from third and fourth pharyngeal pouches
  • D Deficiency of terminal complement components C5 through C9

Why B is correct: The pattern — male infant, onset after maternal IgG wanes (~6 months), recurrent encapsulated bacterial infections (S. pneumo, H. flu), absent tonsils/lymph nodes, virtually absent CD19+ B cells, panhypogammaglobulinemia, and X-linked family history (maternal uncle) — is classic for X-linked agammaglobulinemia. BTK is required to signal through the pre-B cell receptor; without it, B cells arrest at the pre-B stage and never reach the periphery to make antibody.

Why each wrong choice fails:

  • A: NADPH oxidase deficiency causes chronic granulomatous disease, which presents with abscesses and infections by catalase-positive organisms (S. aureus, Aspergillus, Serratia), not encapsulated bacteria. Immunoglobulin levels and B-cell counts are normal in CGD. (The Bug-to-Compartment Map)
  • C: DiGeorge (22q11.2 deletion) causes thymic aplasia and a T-cell deficiency, presenting with viral, fungal, and PJP infections in early infancy along with hypocalcemic tetany and conotruncal cardiac defects. This patient has normal CD3+ T-cell counts and an encapsulated-bacteria pattern. (The Age-of-Onset Sieve)
  • D: Terminal complement (C5-C9) deficiency presents with recurrent Neisseria infections (meningococcemia, disseminated gonococcal infection), not S. pneumoniae or H. influenzae, and immunoglobulin levels and B-cell numbers are normal. (The Bug-to-Compartment Map)
Worked Example 2

Which of the following best describes the immunologic mechanism of this patient's current illness?

  • A IgE-mediated mast cell degranulation triggered by re-exposure to cephalosporin
  • B IgG antibody binding to a cell-surface drug-hapten on circulating red blood cells
  • C Deposition of circulating drug-antibody immune complexes activating complement ✓ Correct
  • D Drug-specific CD4+ T cells releasing IFN-γ and recruiting macrophages

Why C is correct: The 10-14 day latency from drug exposure to onset of fever, polyarthralgias, urticarial rash, glomerulonephritis with low complement, and an elevated ESR is the classic serum-sickness pattern — a Type III hypersensitivity reaction. Soluble drug-antibody complexes deposit in synovium, skin venules, and glomeruli, activating complement (driving down C3/C4) and recruiting neutrophils. Cefazolin and other beta-lactams are common precipitants in modern serum-sickness-like reactions.

Why each wrong choice fails:

  • A: Type I IgE-mediated reactions cause urticaria, bronchospasm, and anaphylaxis within minutes of exposure, not 10 days later, and they do not produce arthralgias, glomerulonephritis, or hypocomplementemia. (The Timing Discriminator for Hypersensitivity)
  • B: A drug-hapten Type II reaction on RBCs causes drug-induced immune hemolytic anemia (positive direct Coombs, falling hemoglobin), not arthralgias and nephritis with hypocomplementemia. The vignette gives no signs of hemolysis. (The Linear-vs-Granular Immunofluorescence Trap)
  • D: Type IV reactions to drugs typically present as contact dermatitis, fixed drug eruptions, or DRESS-type syndromes — not as a multisystem complex of arthralgias, urticarial rash, and immune-complex GN with low complement. (The Timing Discriminator for Hypersensitivity)
Worked Example 3

Which of the following microbial properties best explains why this patient's neutrophils are unable to clear her infections?

  • A The organisms produce IgA proteases that cleave secretory antibody at mucosal surfaces
  • B The organisms generate hydrogen peroxide that the host normally converts to hypochlorous acid
  • C The organisms produce catalase, neutralizing their own hydrogen peroxide before host enzymes can use it ✓ Correct
  • D The organisms have polysaccharide capsules that resist phagocytosis until opsonized by antibody

Why C is correct: This patient has chronic granulomatous disease — abnormal DHR test indicates a defective NADPH oxidase, so neutrophils cannot generate the superoxide-derived hydrogen peroxide needed for the respiratory burst. CGD patients can still kill catalase-negative organisms because those organisms supply their own H2O2 for myeloperoxidase to convert into hypochlorous acid. Catalase-positive organisms (S. aureus, Aspergillus, Serratia, Nocardia, Burkholderia) destroy their own H2O2, leaving CGD neutrophils with no peroxide substrate and allowing the organism to survive.

Why each wrong choice fails:

  • A: IgA protease production is a virulence factor of mucosal pathogens like S. pneumoniae, H. influenzae, and N. gonorrhoeae and is relevant to humoral/secretory immune defense, not to the NADPH oxidase pathway. It does not explain CGD susceptibility. (The Bug-to-Compartment Map)
  • B: This describes the normal myeloperoxidase reaction in healthy neutrophils — converting H2O2 to HOCl. It does not explain why CGD patients are uniquely vulnerable to catalase-positive organisms; it is the function CGD lacks substrate for, not the microbial property.
  • D: Encapsulated organisms cause infections in B-cell and complement deficiencies, not CGD. This patient has normal immunoglobulins and complement, and her infectious agents (S. aureus, Serratia) are characterized by catalase production, not by polysaccharide capsules. (The Bug-to-Compartment Map)

Memory aid

"ACID" for hypersensitivity types: I = Allergic/Anaphylaxis (IgE), II = antiBody Cytotoxic, III = Immune complex, IV = Delayed (T-cell). For phagocyte bugs, remember catalase-positive S-A-N-S-B (Staph, Aspergillus, Nocardia, Serratia, Burkholderia) attack Chronic Granulomatous Disease patients. For complement: "Neisseria needs MAC" — terminal pathway protects against meningococcus and gonococcus.

Key distinction

Type II vs Type III — both are antibody-mediated and both can hit kidney, but Type II antibody binds a fixed tissue antigen (linear IF, e.g., anti-GBM in Goodpasture), while Type III deposits soluble circulating complexes (granular 'lumpy-bumpy' IF, low complement, e.g., post-strep GN, lupus). The IF pattern and the C3/C4 level break the tie.

Summary

Match the timing and effector (IgE, antibody-on-tissue, immune complex, T-cell) for hypersensitivity, and match the bug pattern (encapsulated, opportunistic intracellular, catalase-positive, Neisseria) to the immune compartment that's broken — that two-step recognition wins these items.

Practice hypersensitivity and immunodeficiency adaptively

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Frequently asked questions

What is hypersensitivity and immunodeficiency on the USMLE Step 1 & 2?

Hypersensitivity reactions are categorized into four mechanistic types (I-IV) based on the immune effector causing tissue injury: IgE-mast cell (I), antibody-mediated cytotoxic (II), immune complex (III), and T-cell mediated (IV). Primary immunodeficiencies are classified by which arm of immunity is broken — B-cell (recurrent encapsulated bacteria, sinopulmonary), T-cell (viral, fungal, opportunistic intracellular), combined (everything early in life), phagocyte (catalase-positive bacteria, abscesses), or complement (Neisseria, lupus-like). On exam day, your job is to read three lines of clinical or laboratory pattern and reach for the right bucket — the diagnosis itself is usually the close-mimic distractor.

How do I practice hypersensitivity and immunodeficiency questions?

The fastest way to improve on hypersensitivity and immunodeficiency is targeted, adaptive practice — working questions that focus on your specific weak spots within this sub-topic, getting immediate feedback, and revisiting items you missed on a spaced-repetition schedule. Neureto's adaptive engine does this automatically across the USMLE Step 1 & 2; start a free 7-day trial to see your sub-topic mastery climb in real time.

What's the most important distinction to remember for hypersensitivity and immunodeficiency?

Type II vs Type III — both are antibody-mediated and both can hit kidney, but Type II antibody binds a fixed tissue antigen (linear IF, e.g., anti-GBM in Goodpasture), while Type III deposits soluble circulating complexes (granular 'lumpy-bumpy' IF, low complement, e.g., post-strep GN, lupus). The IF pattern and the C3/C4 level break the tie.

Is there a memory aid for hypersensitivity and immunodeficiency questions?

"ACID" for hypersensitivity types: I = Allergic/Anaphylaxis (IgE), II = antiBody Cytotoxic, III = Immune complex, IV = Delayed (T-cell). For phagocyte bugs, remember catalase-positive S-A-N-S-B (Staph, Aspergillus, Nocardia, Serratia, Burkholderia) attack Chronic Granulomatous Disease patients. For complement: "Neisseria needs MAC" — terminal pathway protects against meningococcus and gonococcus.

What's a common trap on hypersensitivity and immunodeficiency questions?

Confusing Type II and Type III on lupus nephritis (it's III, immune complex) vs Goodpasture (II, anti-GBM)

What's a common trap on hypersensitivity and immunodeficiency questions?

Picking CVID over Bruton when the infant pattern (early onset, absent B cells, X-linked male) clearly favors XLA

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