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phagocytosis
- What is phagocytosis?
- Which cells in the body perform phagocytosis?
- How does phagocytosis help protect the body from disease?
- What are the basic steps involved in phagocytosis?
- What types of objects or particles do cells remove using phagocytosis?
- How does phagocytosis relate to the immune system's response to infections?
phagocytosis, process by whichcells known asphagocytes ingest or engulf other cells or particles. Thephagocyte may be a free-living single-celled organism, such as anamoeba, or one of the body cells, such as awhite blood cell. In some forms of animal life, such as amoebas andsponges, phagocytosis is a means of feeding. In higher animals phagocytosis is chiefly a defensive reaction against infection and invasion of the body by foreign substances (antigens).
Early observations
The presence of foreign particles within cells was first described in the 1860s by pathologist Kranid Slavjansky. In the 1880s Russian-born zoologist and microbiologistÉlie Metchnikoff introduced the termphagocyte in reference toimmune cells that engulf and destroy foreign bodies such asbacteria. Metchnikoff also recognized that phagocytes play a major role in the immune response, a discovery that earned him a share of the 1908Nobel Prize for Physiology or Medicine.
Types of phagocytes
The particles commonly phagocytosed by white blood cells include bacteria, dead tissue cells,protozoa, various dust particles, pigments, and other minute foreign bodies. In humans, and invertebrates generally, the most-effective phagocytic cells are two kinds of white blood cells:macrophages (large phagocytic cells) andneutrophils (a type ofgranulocyte). Macrophages occur especially in thelungs,liver,spleen, andlymph nodes, where their function is to free the airways,blood, andlymph of bacteria and other particles. Macrophages also are found in all tissues as wandering amoeboid cells, and themonocyte, aprecursor of themacrophage, is found in the blood.
The smaller phagocytes are chiefly neutrophils that are carried along by the circulating blood until they reach an area of infected tissue, where they pass through theblood vessel wall and lodge in that tissue. Both macrophages and neutrophils are drawn toward an area of infection or inflammation by means of substances given off by the bacteria and the infected tissue or by a chemical interaction between the bacteria and the complement system of bloodserum proteins. Neutrophils may also engulf particles after colliding with them accidentally.
Particle adherence
Before phagocytosis is accomplished, the phagocyte and the particle must adhere to each other, the possibility of which depends largely on the chemical nature of the surface of the particle. In the case of bacteria, if the phagocyte cannot adhere directly, protein components of the blood known asopsonins (e.g.,complement andantibodies) form a surface film on bacteria—a process known as opsonization. Phagocytes adhere to the opsonins, and phagocytosis follows.
Encapsulated bacteria are ingested with more difficulty. In the absence of specific antibodies that recognize the bacteria, opsonization cannot occur, and the bacteria repel phagocytes. The surfaces of such bacteria are coated with specialantibodies only after the body has mounted an immune response to the presence of that particular kind of bacterium. Such antibodies are of great importance in establishingimmunity to diseases.
Particle engulfment and digestion
The speed with which a phagocyticcell ingests a particle varies somewhat with the size of the particle. Small particles, such as bacteria or minute grains of charcoal, are ingested almost instantaneously. Larger objects, such as clumps of bacteria or tissue cells, are phagocytosed over the course of a more-prolonged response. The cell flows around the object until it has been completely engulfed. The engulfed object is thus enclosed within a membrane-bound vacuole called aphagosome. The phagocytedigests the ingested particle with hydrolytic enzymes, which are contained within membrane-enclosed sacs calledlysosomes found within the cell. Phagocytic enzymes are secreted into the vacuole in which digestion takes place. Small organic components of the particle are used to build larger molecules needed by the cell.
