Fc γ RI (CD64), Fc γ RII (CD32), and Fc γ RIII (CD16) are thethree distinct kinds of IgG Fc receptors that have beenidentified. Humoral immunological responses may bemediated by the low affinity receptors CD64 and CD16.Induced in neutrophils after IFN-γ stimulation, CD64 is asurface glycoprotein that has a high affinity for monomericIgG and is constitutively expressed on monocytes andmacrophages. It is possible that CD64 contributes to thestart of cell-mediated cytotoxicity. Three genes that encodefour different CD64 transcripts have been identified so far.The signal transducing component of the high affinity IgEreceptor has been demonstrated to bind with CD64.Following receptor cross-linking, the Src family kinases Hckand Lyn exhibit elevated kinase activity and willcoimmunoprecipitate with CD64.