Another important innate defense mechanism is the ingestion of extracellular particulate material by phagocytosis. Phagocytosis is a phenomenon in which there is uptake of material by a cell from its environment. In phagocytosis, a cell’s plasma membrane expands around the particulate material to form large vesicles called phagosomes. Most phagocytosis is conducted by specialized cells, such as blood monocytes, neutrophils, and tissue macrophages. Phagocytosis may be enhanced by a variety of factors collectively termed as opsonins which consist of antibodies and various serum components of comple-ment.
Polymorphonuclear (PMN) leucocytes also referred to asgranulocytes include basophils, mast cells, eosinophils and neutrophils. These short-lived phagocytic cells contain lysosomes filled with hydro-lytic enzymes. They play a major role in protection against infection.
Macrophages: These cells enter the blood as monocytes andmigrate to different tissues In these tissues they undergo different changes. The monocyte is a small, spherical cell with few projections, abundant cytoplasm, little endoplasmic reticulum, and many granules. Macroph-ages of different tissues have different names. In liver they are called Kupffer cells, in lungs - alveolar macrophages, in spleen - splenic mac-rophages, in brain – microglial cells etc. Macrophages have the follow-ing functions: 1.They phagocytose particles from the environment,(2) process antigens and present to T cells,thus function as antigen-pre-senting cells.
Certain cytotoxic or killer cells destroy the target cell not by phagocytosis but by releasing biologically potent molecules. Such killer cells include the cytotoxic T lymphocytes and Natural killer cells (NK Cells).
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