Cytokines are singaling soluble proteins which are secreted by the immune system cells and act as intercellular mediators in the generation of immune response, sending messages that are delivered to the same cell (autocrine), an adjacent cell (paracrine), or a distant cell (endocrine). Cytokines take part in reproduction, growth and development, normal homeostatic regulation, response to injury and repair, blood clotting, and immunity and tolerance. The cytokines bind to a specific receptor, provoking a change in function of the target cell.
Based on their function, cell of secretion, or target of action, cytokines can be divided into three groups: interleukins, chemokines, and lymphokines. Interleukin is a protein which is produced by one type of lymphocyte or macrophage and act on other leukocytes. Chemokine is a specific class of cytokines that mediates chemoattraction (chemotaxis) between cells. Lymphokines are produced by a type of immune cell known as a lymphocyte. Lymphokines direct the immune system response by signaling between its cells. Lymphokines play important roles, such as the attraction of other immune cells, like macrophages and other lymphocytes, to an infected site and their subsequent activation prepare them to attack the invaders.
Another important function performed by cytokines is wound healing. Cytokines make sure that the restorative sequences are executed in the appropriate order by signaling blood cells and vascular endothelium to coagulate and fill in a wound opening, recruiting and signaling macrophages and neutrophils to engulf microbes, and guiding protective skin epidermal cells to grow over the wounded area.