ABSTRACT
Leukocytes or other types of cells produce soluble proteins or glycoproteins termed
cytokines
that serve as chemical communicators from one cell to another. Cytokines are usually secreted, although some may be expressed on a cell membrane or maintained in reservoirs in the extracellular matrix. Cytokines combine with surface receptors on target cells that are linked to intracellular signal transduction and second messenger pathways. Their effects may be autocrine, acting on cells that produce them, or paracrine, acting on neighboring cells, and only rarely endocrine, acting on cells at distant sites.
