Cytokines are cell signalling molecules that are vital in mediating the immune response within the body. The cytokines include a wide range of proteins, peptides and glycopeptides.
Cytokines may be categorized into various different groups such as lymphokines, chemokines or interleukins depending on factors such as the type of cell they are secreted from, their function and the target they act upon.
If the primary cell from which they are derived is a monocyte, for example, the cytokine is termed a monokine, while those derived from lymphocytes are called lymphokines.
Interleukin was originally the term used to describe cytokines that were assumed by researchers to primarily be targeting leukocytes. However, it emerged that interleukins did not only target leukocytes and the term is now mainly used to name and number new cytokines as they are discovered. Today, 35 interleukins have been discovered and are numbered 1 to 35 according to the order of their discovery (IL-1 through to IL-35). The interleukins are mainly produced by T-helper cells.
Initially, the term IL-1 was used to define a cytokine from a monocyte and the term IL-2 was used to define a cytokine from a lymphocyte. However, their activities were multiple and overlapping. For example IL-1 causes fever, activates B cells, acts as a co-factor for T cell proliferation in the presence of antigens and induces acute-phase protein synthesis. IL-2 expands T cell proliferation, activates B cells and was initially called T cell growth factor.
Cytokines were termed chemokines if they mediated the chemically-induced movement of body cells known of as chemotaxis or chemoattraction. There is, however, again considerable overlap. IL-8, for example, is actually a chemokine that was named an interleukin.