The immune system is an organization of cells, tissues, organs and molecules with specialized roles in defending against viruses, micro-organisms, cancer cells and non specific proteins.
The cells responsible for both specific and nonspecific immunity are the white blood cells called leukocytes. All of the leukocytes originate from pleuripotent stem cells in fetal liver and in the bone marrow of animal host, from which they migrate to other body sites, undergo further development and perform their various functions. The cells of the immune system are present throughout the host�s body. The different kinds of leukocytes are: Lymphoid cells; Mononuclear cells; Granulocytes; Mast cells and Dendritic cells.
Mononuclear cells
There are two types of mononuclear (i.e., a single large nucleus) cells- monocytes and macrophages.
Both types are highly phagocytic and make up the monocyte � macrophage system.During hematopoiesis in bone marrow, granulocyte monocyte progenitor cells differentiate into promonocytes, which leave the bone marrow and enter the blood where they further differentiate into mature monocytes.
Monocytes are mononuclear phagocytic leukocytes with an ovoid or kidney shaped nucleus and granules in the cytoplasm that stain gray blue. They are produced in the bone marrow and entered the blood, circulate for about eight hours , enlarge migrate to the tissues, and matures into macrophages.
Macrophages are derived from monocytes and are also classified as mononuclear phagocytic leukocytes. However, they may be larger than monocytes, contain more intracellular organelles in both number and complexity (especially lysosomes and phagolysosomes) and have a plasma membrane covered with ruffles or microvilli.
Macrophages have receptors for antibodies and complement: these can coat microorganisms or foreign material and enhance phagocytosis. This enhancement is termed opsonization. Macrophages are dispersed throughout the body. Some take up residence in particular tissues, becoming fixed macophages, whereas others remain motile and free, or wandering, macrophages. Free macrophages travel by amoeboid movement through out the tissues. Macrophage like cells serve different functions in different tissues and are named according to their tissue location:
Alveolar macrophages in the lung
Histiocytes in connective tissues
Kupffer cells in the liver
Mesangial cells in the kidney
Microglial cells in the brain
Osteoclasts in bone
Although in resting state, macrophages are activated by a variety of stimuli in the course of an immune response. Phagocytosis of particulate antigen serves as an initial activating stimulus. However, macrophage activity can be further enhanced by cytokines secreted by activated TH cells, by mediators of the inflammatory response, and by components of bacterial cell walls. One of the most potent activators of macrophages if interferon gamma (IFN-y) sectereted by activated TH cells.
Activated macrophages are more active than the resting ones in eliminating potential pathogens, because they exhibit greater phagocytic activity, an increased to kill ingested microbes, increased secretion of inflammatory mediators, and increased ability to activate T cells. In addition, activated macrophage, but not the resting ones secrete various cytotoxic proteins that help eliminate a broad range of pathogens, including virus infected cells, tumor cells, and intracellular bacteria. Activated macrophages also expresses higher levels of class II MHC molecules, allowing them to function more effectively as anigen presenting cells. Thus, macrophages and TH cells during the immune response facilitate each other�s activation.
