🧬 Hypersensitivity Reactions: The 4 Types, Mnemonics, and Clinical Vignettes
The Gell and Coombs classification of hypersensitivity reactions breaks immune responses into four types. These often appear in Step 1 vignettes where you need to deduce the underlying mechanism based on time of onset, type of immune mediator, or result of a lab test (like Coombs).
Let’s simplify these four with mnemonics, mechanisms, and must-know examples.
🧪📘 Mnemonic: ACID
A - Type I: Anaphylactic
C - Type II: Cytotoxic
I - Type III: Immune Complex
D - Type IV: Delayed (T-cell)
🧪 Hypersensitivity Overview Table
Type | Name | Key Mediators | Example Conditions | Notes |
---|---|---|---|---|
I | Anaphylactic | IgE, Mast cells | Asthma, Anaphylaxis, Allergies | Immediate onset (minutes) |
II | Cytotoxic | IgG/IgM, Complement | Graves, Goodpasture, Hemolytic anemia | Antibodies target host cells |
III | Immune Complex | Ag-Ab-Complement Complexes | SLE, Post-strep GN, Serum sickness | Complexes deposit in tissues |
IV | Delayed (T-cell) | CD4/CD8 T cells | TB skin test, Contact dermatitis, GVHD | Delayed onset (48–72 hours) |
🎯 Fast Clinical Clues
Type I: Sudden onset post-exposure (bee sting, peanuts)
Type II: Direct cell attack (positive Coombs test, hemolysis)
Type III: Immune complex damage (kidneys, joints, serum)
Type IV: TB PPD, poison ivy, graft-versus-host disease
🧠 KOTC Tip: Remember “I and II = antibody, III = complex, IV = delayed cell”*
🧠 Extended Mnemonic: ABCD
Letter | Type | Reaction | Memory Trigger |
---|---|---|---|
A | I | Allergy | Asthma, Anaphylaxis |
B | II | antiBody | Antibody binds cells → cytotoxic |
C | III | immune Complex | Complex deposits → inflammation |
D | IV | Delayed | T-cell reaction (48–72 hr delay) |
📌 Before You Go…
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