Molecules that communicate among cells of the immune system are referred to as cytokine Cytokines are soluble molecules,although some also exist in membrane bound The interaction of cytokine with its receptor on a target cell can cause changes in expression of adhesion molecules and chemokine receptors on the target membrane,thus allowing it to move from location to another. Cytokines can also signal an immune cell to increase or decrease the activity of paicular enzymes or to change its transcriptional program, thereby altering and enhancing its effector functions They can instruct a cell when to survive and when to die Reference: Ananthanarayan and Paniker&;s Textbook of Microbiology Tenth edition