Activating and inhibitory proteins shape the outcome of FcεRI stimulation by different Ag concentrations. The Ag-triggered activation of the FcεRI generates both positive and negative signals, which are integrated to determine the quantity of the response. At every Ag concentration both types of signals are generated, however, their ratio and composition differs depending on the antigen dose. Low to optimal Ag concentrations preferentially engage activating signaling elements (green area and proteins indicated there; the tyrosine kinases Lyn and Syk act proximal to the receptor; Btk, PLC-γ, and STIM1 control Ca2+ mobilization). In response to high, supra-optimal Ag concentrations negative signaling elements are increasingly recruited (red triangle and proteins indicated there; FcεRIβ, Lyn, and SHIP1 are central to the inhibitory signalosome; PKC-δ, Bam32, Cbl-b, and actin transduce suppressive signals at supra-optimal Ag). Note that the SFK Lyn is both an activating and an attenuating element depending on the Ag dose (see text).