Macrophages exposed to Th1 cytokines and PAMPs were described as classically activated (M1) macrophages able to produce NO and fight infection, whereas Th2 cytokines, such as IL-4, IL-10 and IL-13 (23) induce alternatively activated (M2) macrophages, which express Arginase 1 (Arg1) and metabolize L-arginine towards the polyamine pathway (15, 16, 24–26). The gene discussed is ARG1; the disease is infection.