Macrophage

A macrophage is a large white blood cell that engulfs and digests pathogens, dead cells and debris, and that sends signals to organize other immune cells. The name means "big eater," and the cells are a main part of the body's innate defenses.

Some macrophages develop from blood monocytes that enter tissues, and many arise from embryonic precursors and persist in organs for years. They take specialized forms in different places: Kupffer cells in the liver, alveolar macrophages in the lungs, microglia in the brain, and osteoclasts in bone. Élie Metchnikoff first described the engulfing of microbes by such cells, called phagocytosis, in 1882, and shared the 1908 Nobel Prize.

Macrophages recognize microbes through receptors for patterns shared by pathogens, engulf them by endocytosis and destroy them in lysosomes using enzymes and reactive oxygen. They display fragments to T cells using the major histocompatibility complex, release signals such as interferon and cytokines, and clear dead cells. In wound healing they switch to a repair role.

Some microbes live inside macrophages, including the bacterium that causes tuberculosis, HIV and the dengue virus, in which pre-existing antibodies can increase infection of the cells. Macrophages are also involved in chronic inflammation, atherosclerosis and tumors. See immunology.

• Immunology — the science of the immune system
• Endocytosis — the uptake process they use
• Antibody — the molecules that help them recognize targets