Illustration of the Ab response toward Bet v 1a (A) and Bet v1d (B). A, Monomeric Bet v 1a provides only low stimulus to APCs, leading to incomplete APC activation and therefore to a higher induction of Tregs with only moderate Ab levels. Because of the absence of a protective IgG/IgA Ab response, serum IgE levels are sufficient to trigger allergic symptoms. B, As a result of self-aggregation, Bet v 1d is more immunogenic reflected by the induction of higher costimulatory molecules, thereby inducing a strong Th1/Th2-dependent Ab response with high levels of protective IgG/IgA Abs for Ag clearance.