Classifying Injury by Hypersensitivity Type | AI Learning Guide

Use the classification as a decision tool, not a memorized list. Three questions do most of the work. What is the antigen — soluble, fixed to a cell, or an allergen? Which effector is damaging the tissue — IgE and mast cells, antibody plus complement, immune complexes, or T cells and macrophages? And how fast does the injury appear? Minutes points to type one, because only that type uses preformed mediators. Hours with complexes in a filter points to type three. A response that takes a day or more and depends on T cells points to type four. When more than one pathway is active, classify the dominant one, because that is the effector you would need to interrupt.

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The four hypersensitivity types

Type I: IgE on mast cells, allergen cross-linking, minutes, preformed mediators
Type II: antibody against fixed antigen, complement and Fc receptors, localized tissue damage
Type III: immune complex deposition, complement and neutrophil enzymes, injury in vessels and filters
Type IV: T cell mediated, cytotoxic killing or macrophage inflammation, peaks in one to three days

How to classify a scenario

Ask what the antigen is, which effector is causing the damage, and how fast the injury appears. Soluble antigen plus complexes in the glomerulus points to type III. Antibody on a fixed cell surface with complement points to type II. A response that takes two days and depends on T cells points to type IV. The timing alone is often enough to separate type I from the rest, because only type I uses preformed mediators.

Real responses can involve more than one type at once. Classifying the dominant mechanism is still useful, because it identifies the effector that must be interrupted to stop the injury.