Cytokines are small, secreted glycoproteins that fulfill a variety of cellular functions during health and disease. More precisely, cytokines function as regulators and modulators of innate and adaptive immune responses by mediating communication between antigen presenting cells (APCs), lymphocytes and other cells of the immune system. Cytokines act either on cells producing them (autocrine) or on cells in close proximity (paracrine). In case of severe infections, significant amounts of cytokines may also enter the circulation enabling cytokines to act at a distance (endocrine). Effects of cytokines can be agonistic, synergistic, or antagonistic and typically, different cytokines exert similar or overlapping functions, a phenomenon termed functional redundancy. Cell signaling pathways are initiated by binding of cytokines to specific cytokine receptors expressed on the surface of various cell types resulting in a variety of cellular processes including local and systemic inflammation, proliferation and differentiation of antigen-stimulated T and B cells as well as effector functions of T cells. In addition, cytokines induce directed migration of immune cells from blood into tissues and within tissues. These cytokines are classified as chemokines. The significance of cytokines in immune responses is apparent since highly effective drugs targeting cytokines are being developed in order to treat immunologic diseases.