A hypersensitivity reaction to penicillin, present in about 0.5% to 8% of the population. Although different types of hypersensitivity reactions may occur, the most common and potentially dangerous are the type I ( immediate) reactions mediated by immunoglobulin E. If a patient reports a history of signs of local anaphylaxis (e.g., urticaria) or systemic anaphylaxis (e.g., bronchoconstriction, vasodilation) after taking penicillin, no penicillin or other beta-lactam antibiotics (e.g., cephalosporins) should be given to that patient ever again. In those very rare situations in which an infection is susceptible to no other antibiotic and the infection is serious enough to risk the danger of anaphylaxis, the patient may be desensitized with gradually increasing doses of penicillin.