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Cells of The Immune System Monocytes Monocytes, unlike granulocytes, are long lived and can differentiate to become tissue resident macrophages and dendritic cells. Cells of the monocytic lineage play an essential role in initiating and maintaining immune responses by acting as antigen-presenting cells. They process and present antigenic peptides via MHC molecules and express costimulatory molecules such as b7.1 and B7.2. Antigen presenting cells can internalize antigen by phagocytosis, endocytosis, or both. Monocytes circulate in the blood stream for about 8 hours during which time they enlarge, migrate into the tissues and differentiate into specific tissue macrophages which with the neutrophils are the main phagocytes in the body. Monocytes also give rise to dendritic cells which are also migratory cells that can ingest foreign substances but which have a main role as presenters of foreign antigens to lymphoctyes. Granulocytes The granulocytes are classified as neutrophils, eosinophils, or basophils on the basis of cellular morphology and cytoplasmic staining characterisits. Neutrophils, which constitute 50-70% of the circulating white blood cells are much more numberous than eosinophils (1-3%) or basophils (<1%). Granulocytes are innate immune cells that contain specialized granules to provide the first line of immune defense. Granulocytes handle parasites in the lungs and skin, and produce histamine, which stimulates inflammation. Eosinophils are also phagocytic and can migrate from the blood into the tissue spaces. Basophils secrete histamine to help mediate inflammatory reactions and are closely related in function to mast cells. Other Associated Immune Cells Mast cells are formed in the bone marrow during hematopoiesis and released into the blood as undifferentiated precursor cells and do not differentiate until they enter the tissues. They can be found in the skin and mucosal epithelial tissues of the respiratory, genitourinary and digestive tracts. Like basophils, they have large numbers of cytoplasmic granules containing histamine and play an important role in the development of allergies. M cells are specialized cells that transport antigens from the lumina for the respiratory digestive and urogenital tracts to underlying lyphoid organs. NKT cells are a trace population of T cells sharing some features of NK cells, which express an invariant TCRα-chain and respond to glycolipid Ags presented on CD1d. Autologus tissues provide self-glycolipid ligands such that NKT cells are in a chronic state of low level activaiton with suppressor funciton mediated by IL-4. Tumors like melanoma provide glycolipid ligands like GD3. The presentaiton of glycolipid ligand by Cd1d-expressing DCs or the simultaneous delivery of a microbial stimulus to DCs resulting in high IL-12p70 secretion leads in either case to robust activation of NKT cells and IFNy scertion. The critical oeprative is the need for DCs to convert NKT cells from suppressor to effector function. NKT cells rearrange TcR genes and express a CD3/TcR receptor. NK cells do not do this. |
Copyright © 2002-2005 YPatent
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