Glycoproteins secreted by a variety of leukocytes which have effects on other leukocytes (Interleukin = between leukocytes ).
The ultimate purpose of the host defense system is to eliminate invading microorganisms. Once an invading organism is recognized as foreign, elimination is accomplished through phagocytosis and antibody formation (in the case of bacteria ) and cytotoxic attack (for viral , fungal , or other intracellular pathogens). How the immune system recognizes a foreign antigen has been the focus of intense investigation over the years. Immunologists have also been interested in how the immune system forms a long-term "memory" from antigen exposure so that future contact will stimulate an immediate defense against that particular antigen. In recent years they have determined that an important component of immunologic memory is interleukin-2, one of the cytokines produced by the lymphocyte series of white blood cells .

The principal types of interleukins are IL1 , IL2 and IL4 .