Step 1 topics → Immunology → Hypersensitivity Overview and Discrimination

Hypersensitivity Overview and Discrimination

USMLE Step 1 trap: Misassigns effector mechanisms to hypersensitivity types when using the ACID mnemonic. ACID maps as: Type I = Anaphylactic (IgE/mast cells), Type II = Cytotoxic (IgG/IgM + complement/ADCC), Type III = Immune complex (IgG complexes + complement), Type IV = Delayed (T cells, no antibody).

Hypersensitivity overview is one of those topics where knowing each type individually isn't enough — USMLE Step 1 wants you to discriminate between them in context. The exam will give you a clinical vignette and expect you to identify which type is operating based on mechanism, timing, and the effector involved. The ACID mnemonic (Anaphylactic, Cytotoxic, Immune complex, Delayed) is the scaffold, but students consistently fumble it by mixing up which immunoglobulin or cell type belongs to which type — especially Types II and III, which both use IgG and complement.

The trickiest discrimination on USMLE Step 1 is Type II versus Type III. Students see 'IgG + complement' and treat them as equivalent. They're not. The distinguishing question is: where is the antigen? If it's fixed to a cell or tissue surface (like RBCs in hemolytic anemia, or basement membrane in Goodpasture's), that's Type II. If it's floating in circulation as a soluble complex that deposits later (like in serum sickness or lupus nephritis), that's Type III. This fixed-vs-floating framework is what the exam is actually testing when it gives you a case of joint pain, proteinuria, and fever after a drug or infection.

Another reliable trap is serum sickness. Students see 'IgG against a foreign protein' and call it Type II — but the key detail is that the complexes are soluble and circulating before they deposit. That makes it Type III. The exam rewards students who can apply mechanism rather than just pattern-match on 'IgG = Type II.' Build your mental model around effector location and antigen state, not just antibody class.

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Common misconceptions

Common mistake
Wrong: Students mix up which immunoglobulin or effector corresponds to each hypersensitivity type in the ACID mnemonic.
Right: ACID maps as: Type I = Anaphylactic (IgE/mast cells), Type II = Cytotoxic (IgG/IgM + complement/ADCC), Type III = Immune complex (IgG complexes + complement), Type IV = Delayed (T cells, no antibody).
The ACID mnemonic only works if you lock in the effector for each letter. Type I (Anaphylactic) uses IgE bound to mast cells — not IgG. Type II (Cytotoxic) uses IgG or IgM targeting antigens on cell surfaces, activating complement and ADCC. Type III (Immune complex) also uses IgG, but the complexes are soluble and deposit in tissue. Type IV (Delayed) involves no antibody at all — it's purely T cell-mediated, which is why it takes 48-72 hours. Mixing up the effectors, especially confusing IgE for IgG in Type I or forgetting that Type IV has zero antibody involvement, is a direct exam trap.
Common mistake
Wrong: Type II and Type III are interchangeable because both involve IgG and complement activation.
Right: Type II targets antigen fixed to a specific cell or tissue surface, while Type III involves soluble circulating immune complexes that deposit in vessel walls, joints, and kidneys.
Both Type II and Type III use IgG and activate complement, so the antibody class alone doesn't distinguish them — you need to ask where the antigen lives. In Type II, the antigen is anchored to a specific cell or tissue surface (think RBCs, platelets, or glomerular basement membrane), so the immune attack is targeted and localized to that structure. In Type III, the antigen is soluble and circulates as part of an immune complex that later gets trapped in vessel walls, synovial membranes, and kidney glomeruli — causing systemic disease in multiple sites simultaneously. Fixed vs. floating is the discriminating principle.
Common mistake
Wrong: Students classify serum sickness as Type II hypersensitivity because it involves IgG antibodies against a foreign protein.
Right: Serum sickness is Type III because the IgG forms soluble immune complexes with the foreign antigen that circulate and deposit in tissues.
Serum sickness feels like it should be Type II because you can identify specific IgG antibodies directed against a foreign protein — but the classification hinges on what happens after those antibodies form. The antibodies bind the foreign antigen while it's still soluble in circulation, creating circulating immune complexes. Those complexes then deposit in joints, vessels, and kidneys, triggering complement and inflammation at the deposition sites. That sequence — soluble complex formation followed by tissue deposition — is the defining feature of Type III, not Type II, where antibody goes directly to a surface-bound target.
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What the exam tests

  1. Given a clinical vignette or disease example, correctly map it to the right hypersensitivity type using the ACID mnemonic — knowing that Type I = IgE/mast cells (Anaphylactic), Type II = IgG/IgM against cell-fixed antigens (Cytotoxic), Type III = circulating IgG immune complexes (Immune complex), and Type IV = T cell-mediated with no antibody (Delayed).
  2. Distinguish Type II from Type III hypersensitivity in a clinical scenario by identifying whether the antigen is fixed to a cell or tissue surface (Type II) versus floating as a soluble immune complex in circulation that later deposits in tissues (Type III).

Can you avoid these mistakes?

A patient develops fever, arthralgias, and proteinuria 10 days after receiving a horse-derived antitoxin. Which hypersensitivity type is responsible, and what is the key feature of the antigen that determines your answer?
Using the ACID mnemonic, name the antibody class (or absence thereof) and primary effector cell for each of the four hypersensitivity types — without looking. Then check: which two types both involve IgG, and what distinguishes them mechanistically?
A patient with Goodpasture syndrome has IgG antibodies against type IV collagen in the glomerular basement membrane. A different patient with lupus nephritis has granular IgG deposits in the glomeruli. Assign each to Type II or Type III and explain your reasoning using the fixed vs. floating framework.
Contact dermatitis from poison ivy and the tuberculin skin test are both delayed hypersensitivity reactions. What makes them Type IV instead of Type II or III, and what effector is responsible for the tissue damage?

Related topics

Type I Hypersensitivity (IgE / Immediate) Type II Hypersensitivity (Antibody vs Fixed Antigen) Type III Hypersensitivity (Immune Complex) Type IV Hypersensitivity (T Cell / Delayed) Thymus and T Cell Maturation Spleen Architecture and Function

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