🧬 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.

🧬 Hypersensitivity Reactions: The 4 Types, Mnemonics, and Clinical Vignettes

🧪📘 Mnemonic: ACID

A - Type I: Anaphylactic
C - Type II: Cytotoxic
I - Type III: Immune Complex
D - Type IV: Delayed (T-cell)

🧪 Hypersensitivity Overview Table

Hypersensitivity Reactions Tables
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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