Macrophage

Macrophages are leukocytes (i.e., white blood cells) which are part of the cell-mediated immune system. Their major role is enveloping and disposing of particles, which have been "tagged" with chemical attractants called opsonins. They are relatively long-lived in comparison with the other common phagocytic cell, the neutrophil. While neutrophils react quickly to foreign substances or cells, macrophages are slower to respond but live longer, and are a major part of chronic inflammatory response. Their half-life, as circulating cells, is 72 hours, compared to 6 hours for a neutrophil.

After macrophages stop circulating, they become tissue macrophages of various types. An older term for the set of tissue macrophages was the reticuloendothelial system.

Monocyte precursors

Macrophages evolve from leukocytes called monocytes.

Tissue macrophage successors

Tissue macrophages include peritoneal macrophages in the abdominal peritoneum, alveolar macrophages in the respiratory system, histiocytes in soft tissue, Kupffer cells in the liver, and osteoclasts in bone.

Within chronic inflammatory lesions, tissue macrophages may become epitheloid cells, which are characteristic of tissue that displays granulomatous hypersensitivity. Appearing as large, flattened cells with increased endoplasmic reticulum., the epitheloid cells are believed to have differentiated as a result of prolonged antigenic stimulation. Further differentiation or fusion of epithelioid cells is thought to produce multinucleated giant cells.

Activity

While neutrophils arrive at an inflammatory site faster than macrophages, neither can depend on antibody reactions to have taken placed and opsonized the hostile cells. Macrophages, therefore, need an immediate recognition mechanisms. Much as does the complement system, macrophage have, on their surface, proteins that recognize molecules that are not native to the host. Macrocyte recognition molecules are much less specific than immunoglobulins or T-lymphocytes.

Macrophages have 11 types of toll-like receptors that bind to ligands found only on microorganisms. The act of binding triggers a genetically controlled immune response.

Macrophages also recognize mannose molecules, a type of sugar molecule in a spacing that is present only in unicellular organisms. Additional types of surface recognition molecules are coded by known genes; they are normally expressed so nothing needs to enable these mechanisms, in contrast to T-lymphocyte receptors and antibodies.

Some recognition molecules used by macrophages circulate in blood plasma, and are not on the macrophage. C-reactive protein and mannose binding lectin will recognize and bind to foreign molecules, and the bound complex will, in turn, bind to macrophage receptors. These complexes, as well as complement protein C3b, all are opsonins.