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Cytokine is a general name meaning ‘cell produce’. Cytokines are therefore small secreted proteins that mediate and regulate immunity, inflammation, and hematopoiesis normally produced due to an immune stimulus. They act by binding to specific membrane receptors that signal the cell through a second messenger (always enzyme Tyrosine Kinase) that function to switch on or off many cellular functions by altering cellular behavior through gene expression.
The cytokine molecules are characterized by action over a short distance, a short life span, and occurrence at very low concentrations. Their effects on cells include:
- Increase or decrease in the expression of membrane proteins such as the cytokine receptors
- Proliferation of cells
- Secretion of effector molecules which are other cytokines
The other names that are used to refer to cytokines are the lymphokines, those that are produced by the lymphocytes (CD4+ cells) and act on monocytes; monokines those that are produced by monocytes; chemokines which have chemotactic properties; and interleukins which are produced by leukocytes and act on other leukocytes.
In relation to their activity, cytokines are described as having an autocrine action of the affect the cells that produce them, paracrine if they affect the nearby cells to the one that produce them and endocrine if they affect cells that are distant from the cells that produce them. It is very common for different types of cells to produce the same cytokines or for a single cytokine to act on different cell types a process known as pleiotropy. Redundancy occurs when the action of different cytokines results in stimulation of the same function.
Cytokines are often produced in a cascading manner as one cytokine stimulates its target cell to make more cytokines. They can, however, act synergistically where they produce similar effects or antagonistically where they have opposing effects.
their isolation over the years has been quite challenging because of their characteristics such as redundancy, short life span, and occurrence at very low concentrations in circulation. This has however been solved by identifying them at the DNA level and characterizing their activities using the recombinant forms. The predominate producers of these molecules are the CD4+ cells and the macrophages.
There are three categories of Cytokines namely:
- Those that mediate innate immunity
- Those that mediate adaptive immunity
- Those that stimulate hemopoiesis