A patient develops an oral ulcer after 1 week of starting a new antibiotic. This represents which type of hypersensitivity reaction?

A delayed reaction (appearing 1 week after drug exposure) presenting as an oral ulcer is characteristic of Type IV (cell-mediated, delayed-type) hypersensitivity. This reaction is T-lymphocyte mediated and does not involve antibodies. Contact mucositis and drug-induced aphthous-like ulcers from antibiotics fall into this category. Type I is IgE-mediated and occurs within minutes (anaphylaxis, urticaria). Type II involves antibody-mediated cell destruction. Type III involves immune complex deposition. The 1-week delay is the hallmark of Type IV hypersensitivity.