- Cytokines are a family of small proteins that mediate an organism’s response to injury or infection.
- Cytokines operate by transmitting signals between cells in an organism.
- Minute quantities of cytokines are secreted, each by a single cell type, and regulatory functions in other cells by binding with specific receptors.
- Their interactions with the receptors produce secondary signals that inhibit or enhance the action of certain genes within the cell.
- Unlike endocrine hormones, which can act throughout the body, most cytokines act locally, near the cells that produced them.
- Antigen presenting cells and T-cell activation result in rapid intracellular biochemical cascades that induces transcription of many genes including cytokines and their receptors.
- Cytokines are low molecular weight regulatory proteins or glycoproteins secreted by white blood cells and various other cells in the body in response to a number of stimuli.
- Cytokines are usually secreted by cells of the immune system.
- Some cytokines [e.g. type I interferons and tumor necrosis factor (TNF)] are secreted by non-immune cells (e.g. epithelial cells).
- These proteins assist in regulating the development of immune effector cells, and some cytokines posses direct effector functions of their own.
- It includes:
- Monokines: Cytokines produced by mononuclear phagocytic cells.
- Lymphokines: Cytokines produced by activated lymphocytes, especially Th cells.
- Interleukins: Cytokines that act as mediators between leukocytes.
- Chemokines: Cytokines primarily responsible for leucocyte migration.
Mechanism of Action of Cytokines
- Cytokines bind to the specific receptors on the membrane of target cells, triggering signal transduction pathways that ultimately alter gene expression in the target cells.
- Particular cytokines:
- May bind to receptors on the membrane of the same cell that secreted it, exerting autocrine action.
- May bind to receptors on a target cell in close proximity to the producer cell, exerting paracrine action.
- May bind to target cells in distant parts of the body exerting endocrine action.
Function of Cytokines
- On the basis of functions, they can have three biological actions:
- Mediators and regulators of innate immunity: are produced mainly by mono-nuclear phagocytes in response to infectious agents.
- Mediators and regulatory of adaptive immunity: are produced mainly by T lymphocytes in response to specific recognition of foreign antigens.
- Stimulators of hematopoiesis: are produced by bone marrow stromal cells, leukocytes, and other cells, and stimulate the growth and differentiation of immature leukocytes.