Cytokines are secreted multifunctional proteins, exerting local autocrine and paracrine, as well as sys- temic endocrine actions. Although originally thought to be derived from hematopoietic and immune cells, these molecules are secreted by a variety of different cell types and affect multiple target cells. Cytokine actions are characterized by biological pleiotropy and redundancy. Most cytokines affect different biological target cells and act in a cell type specific fashion. Cytokine families often exhibit similar or overlapping functions in a specific target tissue, as they use common receptor subunits and signaling cascades. Different factors, for example, interleukins (ILs), tumor necrosis factors, interferons (IFNs), colony-stimulating factors, chemokines, and others, have been categorized as cytokines, and the definition criteria are some what arbitrary. Cytokines act through specific receptors and use common signaling pathways.