Any of numerous small proteins released from a variety of cell types that affect cell behaviour. Cytokines can influence the cell releasing the cytokine (i.e. autocrine activity), or nearby cells (paracrine activity); in some cases they can enter the bloodstream to influence distant cells (i.e. endocrine activity). Hence, cytokines often behave in a similar manner to growth factors and hormones. Cytokines are crucial to many aspects of cell proliferation, differentiation, migration, and function, and play a central role in immune responses and inflammation. Cytokines produced by lymphocytes are termed lymphokines, and those affecting cell migration, particularly of immune cells, form a large group of chemokines. Cytokines can be classified in several ways, e.g. according to their structure, function, or the type of cell receptor to which they bind when exerting their effect. The main groups include the haemopoietins, some of the interleukins, erythropoietin, colony-stimulating factors, the tumour necrosis factor (TNF) family, and interferons. See also transforming growth factor beta.