 Antigen presentation is a process in which some phagocytes move parts of engulfed materials back to the surface of their cells and "present" them to other cells of the immune system. There are two "professional" antigen-presenting cells: macrophages and dendritic cells. After engulfment, foreign proteins (the antigens) are broken down into peptides inside dendritic cells and macrophages. These peptides are then bound to the cell's major histocompatibility complex (MHC) glycoproteins, which carry the peptides back to the phagocyte's surface where they can be "presented" to lymphocytes. Mature macrophages do not travel far from the site of infection, but dendritic cells can reach the body's lymph nodes, where there are millions of lymphocytes. This enhances immunity because the lymphocytes respond to the antigens presented by the dendritic cells just as they would at the site of the original infection. But dendritic cells can also destroy or pacify lymphocytes if they recognize components of the host body; this is necessary to prevent autoimmune reactions. This process is called tolerance.
