macrophage
(mak'rŏ-fāj‵ 
)
[macro- + -phage]
A monocyte that has left the circulation and settled and matured in a tissue. Macrophages are found in large quantities in the spleen, lymph nodes, alveoli, and tonsils. About 50% of all macrophages are found in the liver as Kupffer cells. They are also present in the brain as microglia, in the skin as Langerhans cells, in bone as osteoclasts, as well as in serous cavities and breast and placental tissue. Along with neutrophils, macrophages are the major phagocytic cells of the immune system. They have the ability to recognize and ingest foreign antigens through receptors on the surface of their cell membranes; these antigens are then destroyed by lysosomes. Their placement in the peripheral lymphoid tissues enables macrophages to serve as the major scavengers of the blood, clearing it of abnormal or old cells and cellular debris as well as pathogenic organisms.
Macrophages also serve a vital role by processing antigens and presenting them to T cells, activating the specific immune response. They also release many substances that participate in inflammation, including chemokines and cytokines, lytic enzymes, oxygen radicals, coagulation factors, and growth factors.
SEE: illus.; chemokine; cytokine; inflammation; oxygen radical.