IL18 and obesity due to melanocortin 4 receptor deficiency: Level 3 confirmation. Human IL-18BP is secreted mostly by hepatocytes and macrophages in the liver. The binding protein binds the pro-inflammatory IL-18 and to a lesser extent also the anti-inflammatory IL-37 cytokine. Inherited IL-18BP deficiency underlies hepatitis by unleashing IL-18 (Belkaya et al., 2019). The IL-18/IL-18BP balance plays a role in several metabolic disorders, such as obesity, diabetes, and atherosclerosis, however, the exact mechanism is yet elusive (Kaplanski, 2018).