🧠 Type I Hypersensitivity Reactions for USMLE Step 1: Mechanism, Triggers, and Clinical Vignettes

Type I hypersensitivity is an immediate allergic reaction mediated by IgE antibodies, most often presenting as:

• Seasonal allergies

• Food or drug-induced anaphylaxis

• Asthma or urticaria

On Step 1, this topic integrates:

• Immune signaling pathways

• Cytokine cascades

• Clinical emergency recognition

🔬 Mechanism of Type I Hypersensitivity

Phase	Event	Key Players
Sensitization	First exposure → IgE production	IL-4, IL-13, plasma cells
Priming	IgE binds mast cells	FcεRI, IgE, mast cells
Re-exposure	Antigen crosslinks IgE → degranulation	Histamine, leukotrienes
Immediate reaction	Vasodilation, bronchospasm	Histamine
Late phase	Inflammation hours later	Eosinophils, IL-5

🚨 Clinical Examples

Condition	Description	Step 1 Buzzwords
Anaphylaxis	Systemic allergic emergency	Hypotension, airway edema, fast onset
Allergic Rhinitis	Seasonal hay fever	Sneezing, eosinophils, watery eyes
Asthma	Bronchospasm from allergens	Reversible airway obstruction
Urticaria	Localized histamine-induced hives	Raised itchy red wheals

💊 Emergency Management of Anaphylaxis

• First-line treatment: Epinephrine IM

  • Alpha effects: ↑ BP, ↓ mucosal edema

  • Beta-2 effects: Bronchodilation

• Supportive care: IV fluids, antihistamines, corticosteroids

📚 Cytokine Summary

Cytokine	Role
IL-4	Stimulates class switching to IgE
IL-5	Activates eosinophils (late phase)
IL-13	Enhances IgE response, mucus production
Histamine	Causes vasodilation, bronchospasm, itching

🧠 Mnemonics to Remember

“I’MEDIATE reaction”

IgE, Mast cell, EDIATE = Immediate response

“A.C.A.U.”

• Anaphylaxis

• Conjunctivitis (allergic)

• Asthma

• Urticaria

⚠️ Step 1 Clinical Clue

A 6-year-old develops swelling, wheezing, and hypotension 10 minutes after a bee sting. What’s the immediate treatment?

✅ Answer: Intramuscular epinephrine
✅ Mechanism: Reverses mast cell effects (vasodilation and bronchoconstriction)

🎯 Call-To-Action

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