Allergic reactions to ß-lactam antibiotics have been reported frequently and may occur because of sensitization to unique haptens or to determinants shared with other drugs. A woman who received 1 tablet of amoxicillin–clavulanic acid developed wheals and fl ares although she had previously tolerated the same preparation well. Levels of specifi c immunoglobulin (Ig) E to penicillin V, penicillin G, amoxicillin, and ampicillin were undetectable. Skin tests to amoxicillin, penicillin major determinant and minor determinant mixture were negative. The patient tolerated oral challenge with 500 mg of amoxicillin but developed wheals and fl ares when challenged with amoxicillin–clavulanic
acid 500/125 mg. A histamine release test was negative with amoxicillin but positive with the amoxicillin–clavulanic acid and clavulanic acid. A prick test to the combination was positive. Specifi c IgE to penicillin V later became positive while remaining negative to other ß-lactams. No inhibition was obtained using penicillin V against clavulanic acid and amoxicillin but was complete when penicillin V was used in the solid-phase and as the inhibitor. No cross-reactivity was proven between these sensitizations.

Key words: Amoxicillin. ß-lactams. Clavulanic Acid. Penicillin V. Sensitization.