Key element of the urea cycle converting L-arginine to urea and L-ornithine, which is further metabolized into metabolites proline and polyamides that drive collagen synthesis and bioenergetic pathways critical for cell proliferation, respectively; the urea cycle takes place primarily in the liver and, to a lesser extent, in the kidneys. Functions in L-arginine homeostasis in nonhepatic tissues characterized by the competition between nitric oxide synthase (NOS) and arginase for the available intracellular substrate arginine. Arginine metabolism is a critical regulator of innate and adaptive immune responses. Involved in an antimicrobial effector pathway in polymorphonuclear granulocytes (PMN). Upon PMN cell death is liberated from the phagolysosome and depletes arginine in the microenvironment leading to suppressed T cell and natural killer (NK) cell proliferation and cytokine secretion (By similarity). In group 2 innate lymphoid cells (ILC2s) promotes acute type 2 inflammation in the lung and is involved in optimal ILC2 proliferation but not survival (PubMed:27043409). Plays a role in the immune response of alternatively activated or M2 macrophages in processes such as wound healing and tissue regeneration, immune defense against multicellular pathogens and parasites, and immune suppression and allergic inflammation; the regulatory outcome seems to be organ specific (PubMed:7537672, PubMed:19360123, PubMed:20483789, PubMed:23552798, PubMed:23637937). In tumor-infiltrating dendritic cells (DCs) and myeloid-derived suppressor cells (MDSCs) plays a role in suppression of T cell-mediated antitumor immunity (PubMed:19414774, PubMed:23248265). H2O + L-arginine = L-ornithine + urea Binds 2 manganese ions per subunit. Nitrogen metabolism; urea cycle; L-ornithine and urea from L-arginine: step 1/1. Homotrimer (By similarity). Interacts with CMTM6 (By similarity). Expressed in macrophages (PubMed:7537672, PubMed:12193690, PubMed:19360123). Expressed in precursor and mature group 2 innate lymphoid cells (ILC2s) (PubMed:27043409). Expressed in lung tumor-associated myeloid cells (PubMed:15313928). Expressed in lung tumor-infiltrating dendritic cells (PubMed:19414774). By T helper 2 (Th2) cytokines such as IL-4, IL-13 and IL-10. In tumor-infiltrating dendritic cells by prostaglandin E2. Belongs to the arginase family.