A patient develops a lip ulcer one week after starting a new medication. What type of immune reaction is responsible?

Correct answer: B. Cell-mediated (delayed, type IV) reaction

• A. Type III reactions (serum sickness, vasculitis) present with fever, arthralgia and purpura, typically 7-14 days but systemically, not as an isolated lip ulcer.
• B. Correct. The one-week latency is the signature of a T-cell-mediated delayed reaction — lichenoid drug eruption and fixed drug eruption are both type IV, driven by cytotoxic T lymphocytes.
• C. IgE reactions occur within minutes to an hour, producing urticaria, angioedema or anaphylaxis — not a delayed ulcer.
• D. Type II reactions target cell-surface antigens causing hemolysis or thrombocytopenia, not mucosal ulceration.