Common Antigenic Determinants of Penicillin G, Cephalothin and 6-Aminopenicillanic Acid in Rabbits¹

The antigenic properties of penicillin G, cephalothin and 6-aminopenicillanic acid were studied in rabbits. The benzylpenicilloyl group appeared to be the major determinant of the penicillin G antigen, in confirmation of earlier reports, but we could not confirm that it was the only determinant involved. Cephalothin and 6-aminopenicillanic acid formed cross-reacting antigens with penicillin G and the cross-reaction was inhibited by benzylpenicilloyl hapten. This suggests the possibility that cephalothin and 6-aminopenicillanic acid become antigenic in vivo through direct conjugation of the β-lactam carbon to native protein and that the penicillins might follow the same pathway.