FCGR2A and temporal arteritis: Consequently, they may act as susceptibility factors for GCA through a variety of mechanisms, such as an inability to bind IgG2 haplotypes (FcγRIIa-131R) on a background of impaired FcγR-mediated phagocytosis (FcγRIIa-131R and FcγRIIIa-158F) and impaired antigen-dependent cellular cytotoxicity of IgG-opsonised cells (FcγRIIIa-158F).