The second major cellular system, the RES, (Fig. 1-1) harbors macrophages, which are cells that play major roles in 1) defending against many microbial pathogens and 2) generating specific immune responses by processing protein antigens and presenting the resultant peptide antigen fragments in the context of MHC molecules to T cells. The system consists of 1) monocytes in the blood, 2) macrophages in the liver, spleen, lymph nodes, thymus, bone marrow, connective tissues, and submucosal tissues of the respiratory and alimentary tracts, 3) dendritic cells in lymph nodes, 4) Langerhans cells in skin, and 4) glial cells in the central nervous system. Macrophages not only operate in direct defense (phagocytosis and intracellular killing) but also marshal other parts of the immune system, such as T lymphocytes (see section on T lymphocytes) (Table 1-1)