Cytokine is a protein molecule released by a cell when activated by an antigen. Cytokines are involved in cell-to-cell communications, acting as enhancing mediators for immune responses (see immune system) through interaction with specific cell-surface receptors on leukocytes. Kinds of cytokines include interleukins (produced by leukocytes), lymphokines (produced by lymphocytes), interferons, and tumor necrosis factor.