Cytokines are a large group of proteins, peptides or glycoproteins that are secreted by specific cells of immune system. Cytokines are a category of signaling molecules that mediate and regulate immunity, inflammation and hematopoiesis. Cytokines are produced throughout the body by cells of diverse embryological origin. Cytokine is a general name; other names are defined based on their presumed function, cell of secretion, or target of action. For example, cytokines made by lymphocytes can also be referred to as lymphokines. Many of the lymphokines are also known as interleukins (ILs), since they are not only secreted by leukocytes but also able to affect the cellular responses of leukocytes. Those cytokines secreted by monocytes or macrophages are termed monokines. And chemokines are cytokines with chemotactic activities.
Cytokines and their receptors exhibit very high affinity for each other. Because of this high affinity, picomolar concentrations of cytokines can mediate a biological effect.
A particular cytokine may exhibit:
Autocrine action by binding to receptor on the membrane of the same cell that secreted it.
Paracrine action binding to receptors on a target cell in close proximity to the producer cell.
Endocrine activity by traveling through circulation and acting on target cells in distant parts of the bod.
Cytokines can regulate cellular activity in a coordinated interactive way due to the following attributes:
Pleiotrophy - one cytokine has many different functions.
Redundancy - several different cytokines can mediate the same or similar functions.
Synergism - occurs when the combined effect of two cytokines on cellular activity is greater than the additive effects of individual cytokines.
Antagonism - the effects of one cytokine inhibits or offsets the effects of another cytokine.