Comparison of cephalosporin and penicillin structures

Cephalosporins and penicillins share a four-membered beta-lactam ring (arrows). Cephalosporins have a six-membered dihydrothiazine ring, while penicillins have a five-membered thiazolidine ring. The R1, and to a lesser extent, the R2 side groups on cephalosporins are important in generating allergenic epitopes.