🧬 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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