ARG1 and obesity due to melanocortin 4 receptor deficiency: Uncontrolled M1 macrophages are associated with acute infections, insulin resistance, and obesity-associated inflammation, which may contribute to the development of diseases including cardiovascular, insulin resistance, fatty liver disease, diabetes, atherosclerosis, and cancer (2, 4–6) In contrast, M2 macrophages are characterized by reduced responsiveness to IL-4 and IL-13, resulting in the induction of low levels of proinflammatory mediators and in the upregulation of arginase 1 (Arg1), CD163, and CD206.