A 28-year-old woman develops severe anaphylaxis 10 minutes after receiving intravenous penicillin for a urinary tract infection. She has no prior history of penicillin allergy. Which of the following characteristics does NOT apply to this adverse drug reaction?

Question

Accepted Answer

A. It is dose-dependent and occurs only at high doses of the antibiotic. ## Type B Adverse Reactions: Penicillin Anaphylaxis

### Classification of Penicillin Anaphylaxis

**Key Point:** Penicillin anaphylaxis is a **Type B (bizarre) adverse reaction**—idiosyncratic, unpredictable, immune-mediated, and **NOT dose-dependent**.

### Characteristics of Type B Reactions

| Feature | Type B (Bizarre) |
|---------|------------------|
| **Mechanism** | Immune-mediated (IgE, T-cell, or complement-dependent) |
| **Dose-dependent** | **NO** — occurs at any dose, even therapeutic doses |
| **Predictability** | Unpredictable; cannot be foreseen from pharmacology |
| **Incidence** | Rare (1–5% for penicillin anaphylaxis) |
| **Relation to drug action** | Unrelated to intended pharmacological effect |
| **Examples** | Penicillin anaphylaxis, chloramphenicol aplasia, sulfonamide Stevens-Johnson syndrome |

### Why Penicillin Anaphylaxis is Type B

1. **Immune-mediated:** Penicillin acts as a hapten, binding to plasma proteins. The immune system recognizes the penicillin-protein complex as foreign and generates IgE antibodies.
2. **Unpredictable:** Cannot be predicted from penicillin's antibiotic action (inhibition of cell wall synthesis). Occurs in genetically predisposed individuals.
3. **NOT dose-dependent:** A patient with penicillin allergy may develop anaphylaxis at a single therapeutic dose or even at trace amounts. High doses are not required.
4. **Rapid onset:** Typically occurs within minutes of exposure (as in this case, 10 minutes).

**High-Yield:** The **absence of dose-dependence** is a critical distinguishing feature of Type B reactions. Type A reactions worsen with increasing dose; Type B reactions occur regardless of dose.

### Clinical Pearl

The patient's lack of prior penicillin allergy history does not exclude Type B reactions—they can occur on first exposure if the patient is genetically susceptible. Skin testing and desensitization protocols exist for patients with documented penicillin allergy who require the drug.

**Mnemonic:** **Type B = Bizarre, not dose-dependent, immune-mediated** (remember: "B is not about dose, it's about biology").

Answer

B. It is immune-mediated (IgE-dependent)

Answer

C. It is unpredictable and unrelated to the drug's pharmacological action

Answer

D. It is a Type B (bizarre) adverse reaction

A 28-year-old woman develops severe anaphylaxis 10 minutes after receiving intravenous penicillin for a urinary tract infection. She has no prior history of penicillin allergy. Which of the following characteristics does NOT apply to this adverse drug reaction?

Explanation

Type B Adverse Reactions: Penicillin Anaphylaxis

Classification of Penicillin Anaphylaxis

Characteristics of Type B Reactions

Why Penicillin Anaphylaxis is Type B

Clinical Pearl

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Feature	Type B (Bizarre)
Mechanism	Immune-mediated (IgE, T-cell, or complement-dependent)
Dose-dependent	NO — occurs at any dose, even therapeutic doses
Predictability	Unpredictable; cannot be foreseen from pharmacology
Incidence	Rare (1–5% for penicillin anaphylaxis)
Relation to drug action	Unrelated to intended pharmacological effect
Examples	Penicillin anaphylaxis, chloramphenicol aplasia, sulfonamide Stevens-Johnson syndrome