Immune system influence on physiology.

The immune system's central function is to maintain homeostasis by guarding the organism against dangerous external and internal stressors. Immunity's operational toolbox contains diverse processes, such as phagocytosis, antigen recognition, cell killing, and secretion of cytokines and antibodies. Although immune cells interact with each other, they also communicate with cells typically associated with other organ systems, including the nervous, circulatory, metabolic, musculoskeletal, endocrine, and hematopoietic. This abundant cross-talk shows that immunity transcends defense and homeostasis: It is a network that participates in many physiological processes necessary for life. By accessing the circulation and inhabiting every tissue, leukocytes sense, interpret, and regulate biological processes. In this Review, we highlight recent studies that illustrate the often bidirectional and symbiotic relationships through which the immune system regulates physiology.