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Cytokines are proteins produced by a variety of cells that function as immunomodulators. Microbial products may alter cytokine expression. Two proteins from Mycobacterium tuberculosis have potential as vaccine adjuvants because they stimulate interleukin-1 and tumor necrosis factor expression. Colony-stimulating factors increase leukocyte proliferation and affect leukocyte function, and granulocyte colony-stimulating factor and granulocyte-macrophage colonystimulating factor have been approved for use in humans by the US Food and Drug Administration. Nonblood cells can produce cytokines and respond to cytokines (eg, acquired immunodeficiency syndrome Kaposi sarcoma cells produce interleukin-6 and interleukin-6 increases Kaposi sarcoma cell proliferation). Cytokines work within networks and can act locally (eg, increases in lung tumor necrosis factor and interferon-γ can limit M. tuberculosis infection). In sepsis the cascade of cytokines can cause a generalized shock reaction. Researchers are pursuing ways to augment the cytokine response at local sites of infection (eg, tumor necrosis factor-a decreases Leishmania major lesion size) and to modulate the septic shock response (eg, anti-tumor necrosis factor-a decreases the severity of murine septic shock).