FCGR2A and autoimmune hemolytic anemia: Thus, while both STIM1 and STIM2 mediate phagocytosis in response to FcγR-activated Ca2+ increase in vitro, the results of FcγR activation in conjunction with C5aR suggest STIM1 as the main signal mediator of the regulatory loop of C5a and FcγRs in AIHA in vivo.