Penicilloic acid
Penicilloic acid is a group of acids formed when penicillin is broken open, usually by enzymes called beta-lactamases. This opening of the penicillin’s beta-lactam ring creates penicilloic acid.
Why it matters: penicilloic acid is the major antigen that drives penicillin allergies. It binds to proteins in the body and acts as a hapten, triggering an immune reaction.
What people experience: about 5% of patients taking penicillin have some allergic reaction, ranging from a simple rash to swelling of the lips or face (angioedema) and even life-threatening anaphylaxis. Among people with mononucleosis treated with ampicillin, the chance of a rash is very high.
Cross-reactivity: people who are allergic to penicillin may also react to other beta-lactam antibiotics.
What doctors do: if there’s a known beta-lactam allergy, clinicians carefully review the severity of the previous reaction before deciding to use another beta-lactam antibiotic.
This page was last edited on 3 February 2026, at 11:33 (CET).