A patient on carbamazepine develops painful blistering skin detachment. This is Stevens-Johnson syndrome — a type B (bizarre, unpredictable) hypersensitivity reaction: mucosa and skin necrose and peel. Stop the drug instantly, intensive supportive care (burns-unit style) — mortality is significant. HLA-B*1502 screening in Asian patients predicts carbamazepine risk — pharmacogenetics preventing the unpredictable. Type B reactions are why every new drug is watched after launch — trials are too small to catch the rare.