Monocytes are an integral part of the innate immune system. In tissues, they can differentiate into macrophages, the gluttons of the immune system. While macrophages perform a variety of functions, one of their most important roles is to eat up, or phagocytose, material around them. They eat dead cells, debris, and, importantly, infectious organisms, a process that is critical for host defense. Macrophages are also the primary infectious target of M. tuberculosis.