Which cephalosporins are associated with higher cross-reactivity with amoxicillin and ampicillin due to similar side chains?

• A. Cefadroxil and Cephalexin only
• B. Cefadroxil, Cephalexin, Cefaclor, Cefprozil
• C. Cefixime and Cefdinir
• D. All cephalosporins have the same cross-reactivity

Answer: (B)

Cross-reactivity between penicillins and cephalosporins mainly depends on side-chain similarity, not just the shared beta-lactam ring. Amoxicillin and ampicillin have a particular amino side chain, and cephalosporins that carry a similar side chain can be recognized by the same immune receptors, increasing the chance of a reaction.

The cephalosporins with side chains that resemble the ampicillin/amoxicillin side chain are cefadroxil, cephalexin, cefaclor, and cefprozil. Because their R1 side chains are similar to those of the aminopenicillins, they’re associated with higher cross-reactivity in patients with those penicillin allergies.

Cephalosporins with different side chains, such as cefixime and cefdinir, tend to have lower cross-reactivity with amoxicillin/ampicillin because their side chains are not as similar to the penicillin side chains. Saying that all cephalosporins have the same cross-reactivity isn’t accurate, since side-chain differences lead to varying levels of risk.