Figure 2:
β-Lactam structure and cross-reactivity β-Lactam antibiotics include penicillins, cephalosporins, carbapenems, and monobactams. Cross-reactivity is possible through the core β-lactam ring, adjacent thiazolidine (penicillin) or dihydrothiazine (cephalosporin) ring, and also from a side chain, R1, or R2 group (left panel). Cephalosporins have both an R1 and R2 group and penicillins only an R1. Despite varied mechanisms, true cross-reactivity is largely based on R1 side chains. Identical side chains in patients with IgE-mediated allergy pose the highest risk. However, cross-reactivity from side chains that are similar, but not identical, and from R2 group similarity is possible and reported. The centre panel demonstrates the structure and rates of cross-reactivity between penicillins, cephalosporins, carbapenems, and monobactams. The right panel details the most clinically important cross-reactivity considerations. *Except for shared group aminopenicillins and cephalosporins. †Monobactams have no shared cross-reactivity with other β-lactams, with the exception for aztreonam and ceftazidime, which share an identical R1. ‡Amoxicillin and ampicillin are structurally similar aminopenicillins and should be considered clinically cross-reactive with each other and the respective cephalosporins with shared R1 groups listed in the figure. Similar considerations exist for the aminocephalosporins.