The elicitation of an immune response to protein antigens depends on the specific recognition of antigenic determinants (epitopes) by T and B lymphocytes. Bee venom phospholipase A2 (PLA) represents the major antigen/allergen of honey bee venom. It displays three dominant immunogenic peptide and one glycopeptide T cell recognition sites. These epitopes are equally recognized by both allergic and nonallergic individuals. A mixture of the three epitope containing peptides was successfully used in specific immunotherapy of bee venom-allergic patients. Both peptide and whole bee venom immunotherapy induced a state of specific anergy in T cells. The production of specific IgE and IgG4 antibodies directly correlated with the secreted interleukin-4:gamma-interferon (IL-4:IFNgamma) ratio, which itself depended on the concentration of available antigen and the strength of the T cell-activating signal. This signal comprises accumulated molecular interactions delivered by engagement of the antigenic peptide/MHC class II complex with the T cell receptor (TcR). Indeed the thermodynamic laws of chemical equilibrium reactions reveal that the antigen concentration, together with the equilibration constant Ki and the related Gibbs standard free energy DeltaG degrees of the MHC-II/Ag/TcR complex reaction, may govern the secreted IL-4:IFNgamma ratio, and in consequence, differential IgE and IgG4 antibody formation. Ki includes epitope and MHC-II haplotype variability and therefore represents a measure of immunological individuality. A major B cell epitope was determined by using point-mutated PLA. Specific antigen recognition by B cells can trigger distinct cytokine profiles in T cells and contribute to the differential regulation of specific IgE and IgG4 antibodies. Our results indicate that distinct cytokine profiles inducing allergic and nonallergic responses can be attributed to thresholds of T cell activation generated by the specific binding properties of individual MHC-II molecules to immunogenic T cell epitopes and their presentation to TcR.