The aim of this article is to analyze the chemical reactivity of cephalosporins resulting in the epitope responsible for recognition by IgE antibodies and to establish the basis of the allergenicity. Increasing evidence supports the role of cephalosporins in IgE hypersensitivity reactions. Third and fourth generation cephalosporins appear to be more involved in specific IgE reactions and often no cross-reactivity with traditional benzyl penicillin determinants exists. In some instances selective responses to unique cephalosporins occur and in others common side-chain similarities exist. Lack of knowledge of the exact chemical structure of cephalosporin antigenic determinants has hindered clinical interpretation of allergic reactions to these drugs and hampered understanding of the specific recognition by IgE molecules of these determinants. Data indicate that R2 is not present in the final conjugate and that recognition by IgE antibodies is mainly directed to the R1 acyl side chain and to the β-lactam fragment that remains linked to the carrier protein in the cephalosporin conjugation process.