The structure for the three human Fc gamma receptors classes Fc gamma RI (CD64), Fc gamma RII (CD32) and Fc gamma RIII (CD16) has been well characterized. Here the IgG binding sites on Fc gamma RII and Fc gamma RII with their responsive FG, BC and C'/E loops on the membrane proximal domains are described in detail. For Fc gamma RI the second extracellular domain is suggested as a key structure of IgG binding. The lower hinge regions of human and murine IgG binding to these Fc receptors and their structural relationship in Fc gamma R-IgG interactions are discussed. The potential of inhibiting the pathophysiological effects of Fc gamma receptors by blocking studies are considered for future therapeutic modalities.