Fig. 1. Characteristic products and functions of M1 and M2 macrophages. Macrophages can metabolize arginine with the inducible nitric oxide synthase enzyme into nitric oxide and citrulline or with arginase into ornithine and urea which is the biochemical basis of the M1 or M2 macrophage responses, respectively.

(a). M1- or M2-dominant macrophages stimulate the Type 1 T helper cell or Type 2 T helper cell responses (b). Also shown are the major molecules involved, including cell surface molecules, cytokines, chemokines and so on, which are closely associated with the M1 or M2 phenotypes (c). Phagocytosis and pinocytosis are general properties of macrophages, which are not dependent on M1 or M2 type responses (d).