Which statement is most accurate regarding cross-reactivity risk across penicillins and cephalosporins?

• A. It is uniform at about 1-2% for all combinations.
• B. It is higher for combinations with similar side chains.
• C. It is impossible to cross-react.
• D. It is higher for combinations with dissimilar side chains.

Answer: (B)

Cross-reactivity between penicillins and cephalosporins is driven mainly by the similarity of their side chains, not by the shared beta-lactam ring alone. The immune system recognizes specific side-chain determinants, so a cephalosporin with a side chain that closely resembles the penicillin that caused the reaction is more likely to trigger a cross-reaction. When the side chains are dissimilar, those shared determinants aren’t present, and the risk drops significantly.

So the statement that cross-reactivity is higher for combinations with similar side chains best captures the main idea. The risk is not uniform across all pairs, it’s not impossible to cross-react, and it’s not higher when side chains are dissimilar.