Fc receptors are proteins found on the surface of certain cells of the immune system including macrophages, monocytes, natural killer cells or B-cells. They allow these cells to bind to antibodies that are attached to the surface of infected cells or pathogens, helping these cells to identify and eliminate pathogens. The functions of Fc receptors includes phagocyte activation or NK cells stimulation that will release cytotoxic molecules to destroy antibody-covered target cells.
Several viral proteins acts as IgG Fc receptors, binding IgG and inhibiting host Fc-dependent immune activation. For instance, Herpes simplex virus type-1 (HSV-1) expresses a Fc receptor (FcR) called gE-gI that is found on the surface of virions and infected cells. gE-gI binds the Fc region of immunoglobulin G (IgG), likely interfering with antibody-mediated viral clearance. Interestingly, the role of mouse cytomegalovirus MCMV encoded FcR was investigated and although essential for viral infection, FcR function is not related to the antibody response. Instead, the viral protein promotes rapid disappearance of host costimulatory protein B7-1 from the cell surface of dendritic cells.