A patient develops a rash two days after beginning a course of amoxicillin. What type of hypersensitivity reaction is this MOST likely to represent?

Correct answer: B. Type IV (delayed, cell-mediated)

• A. Type II reactions involve antibody directed against cell-surface antigens, producing hemolytic anemia or thrombocytopenia — not a delayed rash.
• B. CORRECT. The TIMING is the entire answer. A reaction appearing 48 hours after starting the drug is TYPE IV — delayed, T-cell mediated, requiring sensitized T lymphocytes to be recruited and activated. Morbilliform (measles-like) drug rashes to aminopenicillins are the classic example, and they typically appear days into therapy. Contact dermatitis, the tuberculin skin test and graft rejection share this mechanism.
• C. Type I reactions are IgE-mediated and IMMEDIATE — urticaria, angioedema, bronchospasm and anaphylaxis occur within minutes to an hour of exposure, not two days later.
• D. Type III reactions involve circulating immune complex deposition, producing serum sickness or vasculitis, typically 7–21 days after exposure.

Note: Timing is the diagnostic key: minutes = Type I · days = Type IV. Clinically this matters enormously — a delayed morbilliform rash is not the same as anaphylaxis, and it does not necessarily preclude future penicillin use. Refer for allergy testing rather than labelling the chart permanently.