Phagocytosis is an immune system defense function. An immune cell called a phagocyte ingests and kills invading organisms. Phagocytes also ingest cellular debris caused by tissue injury and dead cells. 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.
Neutrophils
Hematopoietic stem cells in the bone marrow give rise to neutrophils, which are normally the most abundant white blood cells in the circulation. MedlinePlus, a website of the National Institutes of Health, notes that 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 a series of enzymes and other chemicals to kill and digest the organism, as explained in the medical reference text, "Hematology: Clinical and Laboratory Practice." The phagocytic function of neutrophils is the immune system's first line of defense against bacterial infection.
Eosinphils
Eosinophils are closely related to neutrophils; they arise from the same precursor cells. Medline Plus states that eosinophils normally comprise 1 to 4 percent of the circulating white blood cells. Like neutrophils, eosinophils are capable of phagocytosis. The reference text "Hematology: Clinical and Laboratory Practice" points out, however, that eosinophils are not as effective at killing invading bacteria as are neutrophils. Eosinophils have other immune system defense functions including participation in allergic responses and defending the body against parasites.
Monocytes
Monocytes normally comprise 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.
References
- Microbial World: Phagocytes Engulf and Kill Foreign Cells
- Medline Plus: Blood Differential
- "Hematology Clinical and Laboratory Practice"; Rodger L. Bick, M.D., Editor-in-Chief; 1993
- "Robbins Pathologic Basis of Disease, 5th Edition"; Ramzi S. Cotran, M.D., et al., Editors; 1994
- Merck Manual, Home Edition: Components of Blood
