Circadian Clocks and Immune Functions

In mammals, circadian rhythms modulate many physiological processes, including the immune system. This system is divided into two interconnected arms, the innate and the adaptive immune systems. Immunocompetent cells, such as macrophages, natural killer cells, and lymphocytes, have a functional circadian clock. Indeed, daily variations are observed in numbers of circulating cells, as well as in their capacity to secrete cytolytic factors and cytokines. The daily variation is also observed, for example, in lymphocyte expansion after immunization with an antigen across the day and in effector functions against the antigen. In this chapter, we review the current knowledge of circadian rhythms in the immune system, from the first line of immune defense (the innate immune response) to the pathogen-specific control of infections (the adaptive immune response). We end with some examples of immune pathologies influenced by the circadian system.