Interleukins are a family of proteins that regulate the maturation, differentiation, or activation of cells involved in immunity and inflammation, and belong to a broader family termed cytokines. Collectively, these proteins are the key orchestrators of host defense and the response to tissue injury. There are currently 23 different interleukins (numbered from IL-1 to IL-23), although the full extent of the interleukin family will only become clear upon analysis of the human genome sequence. Most importantly, interleukins are central to the pathogenesis of a wide range of diseases that involve an immune component, including conditions such as rheumatoid arthritis, multiple sclerosis, ulcerative colitis, psoriasis, and asthma. Interleukins have also been implicated in other conditions, including cancer, migraine, myocardial infarction, and depression.