Factors inducing MGC formation are less well defined (Helming and Gordon, 2009), but interleukin-4 (IL-4), a TH2 cytokine of alternative (M2) macrophage activation, induces fusion in vitro and in sarcoidosis and foreign body reactions in vivo (Kao et al., 1995, Prokop et al., 2011). Here, IL4 is linked to sarcoidosis.