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Cytokines are soluble peptides that induce activation, proliferation and differentiation of cells of the immune system. Moreover, cytokines influence functions of cells of other tissues and organs, esp. of nervous and endocrine systems. They act in very low concentrations (10-10 M) what makes them to be like hormones. However, hormones tend to be produced constitutively and are produced by endocrine organs. Cytokines, on the contrary, are secreted after activation of particular cells and secretion is short-lived, generally ranging from a few hours to a few days and there are no specialised organs for their synthesis. Cytokines influence target cell in 4 different ways, synergistic, antagonistic, pleiotropic, and redundant way, respectively. They ca act in a autocrine, paracrine and endocrine manner. There are many cytokines that can be divided into those regulating innate and adaptive immunity, to the group of cytokines endowed by chemotactic properties and those supporting growth of hematopoietic and immune system cells.