Cells of the Immune System antigensand witheach other.but also with other cells of the The endothelial cells which line blood vessels form both body.In this context many cells of the body can contribute a barrier and the gateway between the blood and the i.e Within most normal tis ore thertetel cells can be activated by certain cytokines and inflamma membrane-bound cellular fragments derived from mega karyocytes in the bone marrow. ney contain granule cucocytes can are an impor rs at sites molecules which enable them to bind to the walls of noml process within the response.Plat the o rom the b ring the nd endo nortan Erythrocytes play an important role in the clearance of effects upon the immune system.and lymphocytes have antigen-antibody immune complexes from the circulation receptors fora variety of neuropeptidesand hormones.For example. corticosteroid hormones are potent suppressors mmune com hind to th ervthrocvtes.As the ervthrocytes pass through the live spleen.the comp空年aDy Further Readina ne complexes are A variety of cells regulate the development,survival and n DT and RM (1996)The instruc role of amunity in t esponse.Sciene 272: cel tro the In ase.3rd ed the thymus 、ne maturation with receptors that will recognize foreign (rather than self) antigens.Many normal tissue cells throughout the body 0.L01 can be induced to expres MHC class molecule sit Oxford:Blackwells dd(6)Lecture Notes on Immology.3rddn them.depending on the circumstances. ENCYCLOPEDIA OF LIFE SCIENCES/e 2001 Nature Publishing Group /www.els.netantigens and with each other, but also with other cells of the body. In this context, many cells of the body can contribute to the effective functioning of the immune system. The non-nucleated cellular elements of blood, i.e. platelets and erythrocytes, have immunological effects in addition to their other functions. Platelets are essentially membrane-bound cellular fragments derived from mega￾karyocytes in the bone marrow. They contain granules which are an important source of inflammatory mediators (e.g. 5-hydroxytryptamine). They have surface adhesion molecules which enable them to bind to the walls of damaged blood vessels and contribute to the inflammatory response. Platelet-activating factor from activated granu￾locytes and macrophages can also stimulate their degra￾nulation. Erythrocytes play an important role in the clearance of antigen–antibody immune complexes from the circulation in humans and other primates. Following complement activation, C3b adheres to immune complexes which can then bind to complement receptors on the surface of erythrocytes. As the erythrocytes pass through the liver and spleen, the complexes are removed and degraded by macrophages. If immune complexes are not cleared from the bloodstream in this way they can become deposited in tissues (particularly the kidney), leading to tissue damage. A variety of cells regulate the development, survival and migration of B and T lymphocytes. The development of B cells requires interactions with bone marrow stromal cells, and interactions of T-cell precursors with epithelial cells in the thymus promotes the maturation of those with receptors that will recognize foreign (rather than self) antigens. Many normal tissue cells throughout the body can be induced to express MHC class II molecules (particularly at sites of inflammation), enabling them to interact with TH cells either to tolerize them or activate them, depending on the circumstances. The endothelial cells which line blood vessels form both a barrier and the gateway between the blood and the tissues. Within most normal tissues the endothelium forms a tight barrier, preventing anything more than a trickle of white blood cells into the tissues. However, endothelial cells can be activated by certain cytokines and inflamma￾tory mediators to express adhesion molecules to which leucocytes can bind, facilitating their passage through loosened intercellular junctions. This occurs at sites of inflammation (e.g. where the underlying tissues are infected), but is also a normal process within the postcapillary venules of lymph nodes, thus facilitating the passage of T and B cells from the blood into lymphoid tissues during their recirculation around the body. The nervous and endocrine systems have important effects upon the immune system, and lymphocytes have receptors for a variety of neuropeptides and hormones. For example, corticosteroid hormones are potent suppressors of many immune responses. Further Reading Banchereau J and Steinman RM (1998) Dendritic cells and the control of immunity. Nature 392: 245–252. Fearon DT and Locksley RM (1996) The instructive role of innate immunity in the acquired immune response. Science 272: 50–54. Janeway C and Travers P (1997) Basic concepts in immunology. In: Immunobiology: the Immune System in Health and Disease, 3rd edn, pp.1:1–1:30. London: Current Biology/Churchill Livingstone/Gar￾land. Lydyard P and Grossi C (1998) Cells involved in the immune response. In: Roitt I, Brostoff J and Male D (eds) Immunology, 5th edn, pp. 13– 30. London: Mosby. Reeves G and Todd I (1996) Lecture Notes on Immunology, 3rd edn. Oxford: Blackwell Science. Scientific American (1994) Life, Death and the Immune System: Scientific American: A Special Issue. New York: WH Freeman. Cells of the Immune System ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net 7