secreted by a variety of 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 formation
(in the case of ) and cytotoxic attack (for , , or other intracellular pathogens).
How the immune system recognizes a foreign 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 series of .
The principal types of interleukins are ,