Endogenous and exogenous cytokines: An overview and introduction.

Cytokines are ubiquitous small proteins, secreted by virtually all leukocytes and other immune effector cells, that interact with other cytokines and effector and regulatory cells to direct innate and adaptive immunity. Six broad categories of cytokines have been described, with functions ranging from stimulation of immunity and inflammation by cytokines produced by white blood cells (interleukins) to impacting the migration of immune effector cells to sites of inflammation and tissue injury (chemokines). When secreted in excess, or when there exists an imbalance between proinflammatory and anti-inflammatory cytokines, diseases ranging from sepsis to organ transplant rejection to autoimmune disorders occur. The development of exogenous cytokines for therapeutic use, similar or identical to naturally occurring cytokines, has resulted in advances in the management of cancer and autoimmune diseases. On the other hand, the development of inhibitors of cytokines has resulted in the ability to control of a growing list of inflammatory, neoplastic, autoimmune, and allergic conditions. Future investigations should continue to explore the manner cytokines are exploited for therapeutic purpose or are used as inhibitors to interrupt the pathobiological mechanisms of disease.