In a patient allergic to penicillin, a severe anaphylactic reaction developed on first exposure to cephalothin. There is evidence to suggest that the cross-reactivity of cephalothin and penicillin occurs through the penicilloyl determinant. Therefore, patients who are sensitive to penicillin and who exhibit positive penicilloyl-polylysine skin tests have a greater risk of reaction to cephalothin therapy.
Scholand JF, Tennenbaum JI, Cerilli GJ. Anaphylaxis to Cephalothin in a Patient Allergic to Penicillin. JAMA. 1968;206(1):130-132. doi:10.1001/jama.1968.03150010078026