Penicilline,  the prototypic beta-lactam antibiotic, is the most frequent cause of anaphylaxis in humans. The reaction occurs when the compound is covalently bound to tissue carrier proteins to form drug-protein conjugates known as haptens. Ninety-five percent of tissue- bound penicillin is haptenized as benzylpenicilloyl, termed the major antigenic determinant. IgE antibodies directed at this hapten appear to be implicated in type I hyperensitivity reactions.