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Several different cell types coordinate their efforts as part of the immune system, including B cells, T cells, macrophages, neutrophils, basophils and eosinophils. Each of these cell types has a distinct role in the immune system, and each communicates with other immune cells using secreted factors called cytokines, including interleukins, TNF, and the interferons. Macrophages phagocytose foreign bodies and are antigen-presenting cells, using cytokines to stimulate specific antigen dependent responses by B and T cells and non-specific responses by other cell types. T cells secrete a variety of factors to coordinate and stimulate immune responses to specific antigen, such as the role of helper T cells in B cell activation in response to antigen. The proliferation and activation of eosinophils, neutrophils and basophils respond to cytokines as well. Cytokine communication is often local, within a tissue or between cells in close proximity. Each of the cytokines is secreted by one set of cells and provokes a response in another target set of cells, often including the cell that secretes the cytokine. Some cytokines, like IL-1, interferons and TNF, stimulate a broad inflammatory response in response to infection or injury. Other cytokines have more specific functions such the following examples. IL-2 stimulates the proliferation and activation of B and T cells. IL-4 plays a role in the differentiation of Th2 cells, in allergic responses, and in the switching of antibody types. IL-5 stimulates the production and maturation of eosinophils during inflammation. IL-8 is a chemokine, a chemotactic factor that attracts neutrophils, basophils and T cells to sites of inflammation. IL-12 and IL-18 are involved in helper T cell differentiation. IL-10 apparently acts to repress secretion of proinflammatory cytokines. The complex interplay of these different cytokine functions with immune cells is essential for correct immune function. (This definition may be outdated - see the DesignNote.)
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