Cytokines have important roles in chemically induced tissue damage repair, in cancer development and progression, in the control of cell replication and apoptosis, and in the modulation of immune reactions such as sensitization. Each cytokine has a matching cell-surface receptor. Subsequent cascades of intracellular signaling then alter cell functions. Cytokines are often involved in several developmental processes during embryogenesis, which are crucial for fighting off infections and in other immune responses. Cytokines constitute the single most important group of biopharmaceutical substances. As coordinators of the immune and inflammatory response, manipulation of cytokine activity can have a major influence on the body's response to a variety of medical conditions.