Cytokines are signaling proteins, usually less than 80 kDa in size, which regulate a wide range of biological functions including innate and acquired immunity, hematopoiesis, inflammation and repair, and proliferation through mostly extracellular signaling. They are secreted by many cell types at local high concentrations and are involved in cell-to-cell interactions, have an effect on closely adjacent cells, and therefore function in a predominantly paracrine fashion. They may also act at a distance by secretion of soluble products into the circulation (endocrine or systemic effect) and may have effects on the cell of origin itself (autocrine effect). The first group of cytokines described was immune-response factors, labeled as lymphokines that were soluble products released from activated lymphocytes in response to specific or polyclonal antigen. Other groups of cytokines were named according to their actions such as the colony stimulating factors (e.g., granulocyte colony stimulating factor, G-CSF or granulocyte-macrophage colony stimulating factor, GM-CSF), the chemoattractants now grouped under chemokines with a special nomenclature (e.g., CCL11/eotaxin or CXCL1/MIP-1α), or the growth factors that mediate proliferation, differentiation, and survival of cells (e.g. transforming growth factor-β, TGF-β). However, the effect of a particular cytokine is not restricted to only one set of biological functions but often has more than one function extending beyond the function that may be implied in its name. The wide pleiotropy and element of redundancy in the cytokine family, with each cytokine having many overlapping functions, and with each function potentially mediated by more than one cytokine make the classification of cytokines a difficult task.