Phagocytosis refers to ingestion of microorganisms or any particulate material by a cell. It is also a method of nutrition of some protozoa such as Amoeba, but here the mechanism is related to the defense mechanism of body provided by white blood cells through phagocytosis.

Blood consists of fluid known as plasma which contains different constituents such as erythrocytes or red blood corpuscles (RBC), leukocytes or white blood cells (WBC) and thrombocytes or platelets. The leukocytes can be divided on the basis of granules in their cytoplasm, into granulocytes and agranulocytes. Granulocytes contain three types of blood cells ( neutrophils, basophils and eosinophils) and agranulocytes contain two types of cells (lymphocytes and monocytes).

The granules of cytoplasm can be observed under the microscope. After staining here granules take different stains with a mixture of acidic (eosin) and basic (methylene blue)dyes,the granules of neutrophils take red and blue stains. Granules or basophils stain blue with methylene blue, and that of eosinophils stain red with eosin. Neutrophils can enter an infected tissue and kill microorganisms and foreign particles. The basophils can release substances like heparin (an anticoagulant) and histamine (in inflammation and allergic responses).

Eosinophils are the phagocytes. After microbial infection or hypersensitivity their number increases. In agranulocytes the granules are absent. These are of two types, lymphocytes and monocytes.

The lymphocytes are of two types, B-lymphocytes and T-lymphocytes. The B-lymphocytes derived its name from its site of maturation in the bursa of fabricius in birds. The B- lymphocytes depend on the activity of bursa tissues, where as the T lymphocytes (derive its name from thymus) depend on the thymus for their activity. Thymus contain T cell but not B cells, similarily bone marrow consists of only B cells but not T cells. Both the lymphocytes occur in lymphoid tissues ( tonsils, lymph nodes, spleen, thymus gland, thoracic duct, bone marrow, appendix, lymph nodes in respiratory-gastrointestinal and reproductive tracts). The T and B cells cooperate in the presence of a third cell, Mechnikov macrophage. These provide immunity.

Monocytes mature into macrophages and act as phagocytes. The leukocytes are derived from stem cells in bone marrow and enter the lymph system (lymph node, spleen, thymus etc.)

The phagocytes are of two types, granulocytes (microphages) and monocytes (macrophages). When a microbe infects granulocytes (neutrophils), monocytes move to the infected areas. During migration, monocytes enlarge in size and called macrophages. Since these macrophages are migratory, they are also termed as wandering macrophages. Some macrophages remain at a fixed position e.g. in the liver (kupffer's cells), lungs (aleolar macrophage), nervous system (microglial cell), branchial tissue, bone marrow, spleen, and lymph nodes and peritoneal cavity surrounding abdominal organs. These macrophages are called fixed macrophages which constitute the mononuclear phagocytic system.

Mechanism of Phagocytosis

After infection the number of WBC increase in blood during the initial phase of infection. At this stage they are phagocytic in nature. The mechanism of phagocytosis is-

Chemotaxis. It is a phenomenon of chemical attraction of phagocytes to microorganisms. The chemotactic chemicals which attract the phagocytes are the components of WBC, and damaged cells, peptides derived from complements and microbial products.

Attachment. The plasma membrane of phagocyte gets attached to the surface of a microbe or foreign material. When there is a large capsule M protein attachment can be hampered. The large sized microorganisms or foreign material is trapped in blood clots, blood vessels or fibres of connective tissues. If the cell wall of microorganisms is coated with certain plasma protein promoting the attachment of microbe to phagocytes, only then they can phagocytozed. The coat protein are called opsonins and the process of coating of plasma protein is known as opsonization.

Ingestion. After attachment the plasma membrane of phagocyte extends short projections known as pseudopods which engulf the microorganisms or foreign materials. This process is known as ingestion.

Digestion. After engulfment phagosome comes in the contact of lysosome that contains digestive enzymes and bactericidal chemicals. After making contact the membrane of phagosome and lysosome gets fused and a single layered large structure is formed which is called phagolysosome. Within 10-30 minutes the contents of phagolysosomes degrade the microorganisms or foreign materials.

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