Final answer:
People with a PCN (penicillin) allergy have a higher risk of allergenic cross-reactivity with cephalosporins, due to similar β-lactam rings in their structures.
Step-by-step explanation:
A PCN allergy, which is an allergy to penicillin-class antibiotics, is associated with a high risk of allergy to another class of antibiotics known as cephalosporins. Penicillins and cephalosporins both belong to the larger group of antibiotics called β-lactam antibiotics. This is due to the similarity in their chemical structure, mainly the presence of the β-lactam ring which is a cyclic amide with two carbons between the O and N atoms. Because of this structural similarity, people who are allergic to penicillin may potentially react to cephalosporins as well, although cross-reactivity rates are lower with newer generations of cephalosporins.