Cytokines are important signaling molecules in the immune system. The immune processes controlled by cytokines will be introduced in this overview, which covers molecules termed `immunomodulatory cytokines' in this database as well as other cytokines and peptide hormones. The immune response is modulated by cytokines that affect cell proliferation (IL-2, IL-4, growth hormone, TGF♀, and many others), survival (IL-7, prolactin, FasL), differentiation (IL-4, IL-12), antigen presentation (GM-CSF, IFN , IL-10) and trafficking (chemokines). In addition to the direct effects of cytokines on mature cells of the immunesystem, the development of immune cells depends on cytokines (IL-7, SCF, Flt-3L, SDF, IL-15) as does the architecture of lymphoid organs (LT, TNF). Cytokines link the immune system with the inflammatory response (IL-1, IL-6, TNF) and with the central nervoussystem (growth hormone, prolactin, IL-1). Viruses produce a number of agents that combat cytokine pathways (IFN, TNF, IL-1, chemokines) or suppress (IL-10) processes involved in antiviral immunity.