Cytokines are small proteins that act on cells of various types with receptors of their own for each of them. Secreted by lymphocytes (white blood cells involved in cellular immunity) and macrophages (defence cells responsible for digesting foreign particles), they are involved in the development and regulation of immune responses. Some cytokines have been given the name of their main function (interferons, TNF or "tumour necrotizing factor"); others have the generic name of interleukin, followed by a number (from 1 to 13).