The immune system employs several tactics to protect the body. Phagocytosis is one such tactic wherein special types of cells called phagocytes ingest and kill invading organisms. Phagocytes also ingest dead cells and debris caused by tissue injury. Three types of white blood cells can act as phagocytes: neutrophils, eosinophils and monocytes. These white blood cells circulate in the bloodstream awaiting chemical signals of invasion and inflammation. Upon receiving these signals, the white blood cell phagocytes migrate to the site of infection.
Special stem cells in the bone marrow give rise to neutrophils, which are normally the most abundant white blood cells in the circulation. Neutrophils typically comprise 40 to 60 percent of the white blood cells in the circulation. The primary function of neutrophils is phagocytosis. Special features of these cells reflect their adaptation for phagocytic function. The neutrophil surface has proteins that help it attach to an invading organism. The neutrophil then surrounds the invader, engulfing it. Once the organism is engulfed, the neutrophil releases enzymes and other chemicals to kill and digest the organism. The phagocytic function of neutrophils is part of the immune system's first line of defense against bacterial infections.
Eosinophils are closely related to neutrophils. They arise from the same precursor cells in the bone marrow. Eosinophils normally comprise 1 to 4 percent of the circulating white blood cells. Like neutrophils, eosinophils are capable of phagocytosis. However, eosinophils are not as effective at killing invading bacteria as neutrophils are. Eosinophils have other immune system defense functions, including participation in allergic responses and defending the body against parasites, such as intestinal worms.
Monocytes normally represent 2 to 8 percent of the circulating white blood cell population. Like neutrophils, these white blood cells can respond to a site of infection and phagocytize invading bacteria. Monocytes also migrate into the body tissues, where they grow and transform into cells called macrophages. Macrophages remain in the tissues. They continue to function as phagocytes in the tissues, and interact with other types of immune cells to protect the body and regulate inflammatory responses.