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Alpha

Macrophages, T lymphocytes, other cell types, mast cell granules

Initiation of inflammatory response; cytotoxicity for some tumor cells; regulation of certain immune functions; induce fever; chemotactic for granulocytes

Beta

T lymphocytes

Killing of target cells by T cytotoxic cells and natural killer (NK) cells

host defenses have receptors for chemokines, thereby enhancing their ability to migrate to the appropriate region of the body, such as an area of inflammation. Two chemokine receptors, CCR5 and CXCR4, play a critical role in HIV infection; they serve as co-receptors for the virus, influencing which cell types are most likely to become infected. ■ chemotaxis, p. 65 ■ HIV co-receptors, p. 745

■ Colony-stimulating factors (CSFs) are important in the multiplication and differentiation of leukocytes (see figure 15.4). During the immune response when more leukocytes are needed, a variety of colony-stimulating factors direct immature cells into the appropriate maturation pathways.

■ Interferons (IFNs) are glycoproteins important in the control of viral infections. In addition to being antiviral, IFN-gamma helps regulate the function of cells involved in the inflammatory response, particularly mononuclear phagocytes, and modulates certain responses of adaptive immunity. The role of interferons in the containment of viral infections will be described in more detail later in the chapter. ■ glycoproteins, p. 29

■ Interleukins (ILs) are produced by leukocytes; at least 18 interleukins with various functions have been studied. As a group, interleukins are important in both innate immunity, including the inflammatory response, and in adaptive immunity. Their activities often overlap.

■ Tumor necrosis factors (TNFs) were discovered because of their activities in killing tumor cells, which is how they acquired their name, but they actually have multiple roles. TNF-alpha, which is produced by macrophages and other cell types, plays an instrumental role in initiating the inflammatory response. Tumor necrosis factors can also initiate the process of programmed cell death, or apoptosis. ■ apoptosis, p. 387

Groups of cytokines often act together to facilitate a particular response by the host defenses. For example, cytokines referred to as pro-inflammatory cytokines contribute to inflammation (TNF-alpha, IL-1, IL-6, and others). Others are especially involved in promoting antibody responses (IL-4, IL-5, IL-10, and IL-14). A different group promotes responses that involve certain groups of T cells (IL-2, and IFN-gamma, and others).

15.5 Sensor Systems 381

Adhesion Molecules

Adhesion molecules on the surface of cells allow those cells to adhere to other cells. Some cells use adhesion molecules to "grab" other cells as they pass by. For example, when phago-cytic cells in the blood are needed in tissues, the endothelial cells, which are the cells that line the blood vessels, synthesize adhesion molecules, snaring passing phagocytic cells. This slows down the rapidly moving phagocytic cells, and provides them with the opportunity to exit the bloodstream. Other types of adhesion molecules allow cells to make direct contact with one another, thereby enabling cells to target the delivery of cytokines or other compounds to a particular cell.

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