IGHE and response to penicillin: When side chains differ, the risk of cross-reactivity is very low, whereas identical or closely related side chains (for example between ampicillin and cephalexin) can confer a higher risk (32), but even in those with proven IgE- or T-cell-mediated penicillin allergy, cross reactivity with cephalosporins remains as low as 16.45% (33).