A 28-year-old woman presents with acute onset urticaria, angioedema, and bronchospasm 15 minutes after receiving intravenous penicillin for a respiratory tract infection. Which of the following is the most common type of hypersensitivity reaction responsible for this clinical presentation?

Question

Accepted Answer

A. Type I hypersensitivity (IgE-mediated immediate). ## Type I Hypersensitivity — The Culprit

**Key Point:** Type I hypersensitivity is an IgE-mediated, immediate reaction occurring within minutes of antigen exposure, manifesting as urticaria, angioedema, bronchospasm, and potentially anaphylaxis.

### Mechanism
The clinical presentation—acute onset within 15 minutes, urticaria, angioedema, and bronchospasm—is pathognomonic for Type I hypersensitivity:

1. **Sensitization phase**: Prior exposure to penicillin → IgE antibodies bind to mast cells and basophils
2. **Re-exposure**: Antigen cross-links IgE → immediate degranulation
3. **Mediator release**: Histamine, tryptase, leukotrienes, prostaglandins → vasodilation, smooth muscle contraction, increased vascular permeability

### Clinical Features of Type I
- **Onset**: Seconds to 30 minutes (this case: 15 minutes)
- **Manifestations**: Urticaria, angioedema, pruritus, bronchospasm, wheezing, hypotension, anaphylaxis
- **Most common cause**: Drug allergy (penicillin, cephalosporins), food allergy, insect venom, latex

**High-Yield:** Penicillin is the **single most common drug cause** of Type I hypersensitivity and anaphylaxis in clinical practice.

### Why Not Other Types?
- **Type II (cytotoxic)**: Occurs over hours to days; presents with hemolytic anemia, thrombocytopenia, not acute urticaria
- **Type III (immune complex)**: Occurs 3–10 days after exposure; presents with serum sickness, vasculitis, arthralgia—not immediate urticaria
- **Type IV (delayed-type)**: Occurs 24–72 hours later; presents as contact dermatitis or drug rash—not immediate anaphylaxis

**Clinical Pearl:** The **15-minute timeline** is the diagnostic linchpin—only Type I fits this temporal profile.

### Mnemonic for Hypersensitivity Types
**ACID**:
- **A**ntibody-mediated (Type II)
- **C**irculating immune complex (Type III)
- **I**mmediate (Type I)
- **D**elayed (Type IV)

Answer

B. Type III hypersensitivity (immune complex)

Answer

C. Type IV hypersensitivity (delayed-type)

Answer

D. Type II hypersensitivity (cytotoxic)

A 28-year-old woman presents with acute onset urticaria, angioedema, and bronchospasm 15 minutes after receiving intravenous penicillin for a respiratory tract infection. Which of the following is the most common type of hypersensitivity reaction responsible for this clinical presentation?

Explanation

Type I Hypersensitivity — The Culprit

Mechanism

Clinical Features of Type I

Why Not Other Types?

Mnemonic for Hypersensitivity Types

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