Fever is a common response to infection and to other challenges to host defense. Temperature elevation has been associated with effects on the recognition, recruitment, and effector phases of the immune response. Specific immunologic responses are generally enhanced in the setting of temperature elevation within the physiologic range but not the supraphysiologic range. In contrast, natural immune responses may be unchanged or adversely affected. Temperature elevation appears to affect primarily the phase of recognition and sensitization or activation of mononuclear leukocytes. T lymphocyte responses (and/or the interactions of T lymphocytes with monocytes-macrophages) are enhanced for generation of effector cells. The activities of the effector cells, once generated, are usually not enhanced--or may even be depressed--by temperature elevation, but decreases are more than offset by increased T helper function. Overall, the data suggest that temperature elevations of the febrile response constitute a beneficial component of effective host defense.