The majority of human subjects who receive subcutaneous allergen immunotherapy (IT) develop decreased sensitivity to their allergens. Multiple factors may explain the efficacy of IT, some evidence support a role for allergen specific IgG antibodies. There is controversy whether such antibodies act by blocking allergen binding to IgE or initiation of active inhibitory signaling through low affinity IgG receptors (FcgammaRIIB) on mast cells and basophils. In this study, we addressed this question using peripheral blood from cat non-allergic, cat allergic, and immunotherapy-treated cat allergic subjects. Blood from subjects who received IT contain IgG antibodies that mediate inhibition of basophil activation by a mechanism that is blocked by antibodies specific for the inhibitory IgG receptor FcgammaRIIB. Surprisingly, inhibition was also blocked by aglycosylated, putatively non-FcR binding, antibodies that are specific for the FcgammaRIIA, suggesting a contribution of this receptor to the observed effect. Consistent with a cooperative effect, ex vivo basophils were found to express both IgG receptors. In other studies we found that basophils from subjects who were both chronically exposed to allergen and were producing both cat allergen specific IgE and IgG, are hyporesponsive to allergen. These studies confirm that IgG antibodies produced during IT act primarily by stimulation of inhibitory signaling, and suggest that FcgammaRIIA and FcgammaRIIB function cooperatively in activation of inhibitory signaling circuit. We suggest that under normal physiologic conditions in which only a small proportion of FcepsilonRI are occupied by IgE of a single allergen specificity, FcgammaRIIA co-aggregation may, by providing activated Lyn, be required to fuel activation of inhibitory FcgammaRIIB function.