A type of leukocyte that has many functions in protecting the body from disease. Macrophages primarily target antigens that are circulating freely in the blood but also attack cells infected by virus or bacteria and cancer cells. They kill by literally engulfing their targets and digesting them. Macrophages also have an important role in the immune response. Once a macrophage has digested an antigen it stimulates the immune response by presenting part of the antigen to lymphocytes. They also release cytokines that enhance CD8 cell function. HIV can infect macrophages inhibiting these important functions.