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?

## 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").