• amoxicillin;
  • benzylpenicillin;
  • IgE antibodies;
  • side chain

To determine the influence of the acyl side chain on the IgE antibody specificity of the two most common penicillins inducing allergic reactions in our community, benzyl-penicillin (BP) and amoxicillin (AX), IgE positive sera from a group of 24 patients allergic to penicillin were studied. RAST was determined in parallel to benzyl penicilloyl-polylysine (BPO-PLL) and amoxicilloyl-polylysine (AX-PLL) in each serum, and values greater than an established coefficient of variation of 15% were considered as different for either of the haptens used (BP and AX). 16 sera proved to be more positive to BP (Group A), six to AX (Group B) and in two IgE was positive to both haptens with a similar value (Group C). RAST titration concentration effect curves and RAST inhibition studies with a pool of sera from each group (A, B, C) and individual sera showed that most of them were more specific for either BPO or AX, a minority being of similar value to both. These findings indicate that the side chain structure of penicillins is relevant in the constitution of the antigenic determinant, although in some instances the common chemical structure of betalactam is recognised mainly by the IgE antibodies. The clinical relevance of these data needs to be established.