The modulation of metabolism and immune functions, essential in the interaction and susceptibility to infectious diseases, is also regulated by arginine [146]. Arginine is a precursor for the synthesis of proteins, nitric oxide, urea, polyamines, proline, glutamate, creatine and agmatine. The role and relationships between the pathways of arginine synthesis and catabolism are complex, due to the compartmentalized expression of various enzymes in different organs (e.g. liver, small intestine and kidney) and subcellular compartments (cytosol and mitochondria), as well as changes in gene expression in response to diet, hormones and cytokines [147]. As reported by several studies, arginine is the precursor of macrophages and it is now clear that arginine metabolism of immune cells is particularly involved in cancer, inflammation, infections, fibrotic diseases, pregnancy and the regulation of immune system [148, 149]. Macrophage arginine metabolism influences the outcome of immune responses in which innate immune cells are involved. Arginine supplementation is reported to increase T lymphocyte response and Th cell numbers, suggesting a possible role in prolonged or repeated infection [150]. The importance of arginine metabolism as a new field of investigation includes that its depletion delays the growth of some types of tumours, while others report that its integration improves anticancer effects, probably by ameliorating immune function [150].