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Cytokines are small proteins that are secreted by certain cells to regulate immunity and inflammation within the body. They act on their target cells by binding to specific proteins found on the cell membrane, called receptors. Each cytokine can only bind with a specific cytokine receptor, which regulates the activities of both the cytokines and cells being influenced.
There are many different types of cytokines and cytokine receptors. They are categorized based on the type of cell that makes the cytokines and the structure and function of both the cytokine and the cytokine receptor. The largest category of cytokines stimulates the multiplication and differentiation of the different cells involved in an immune response. It is mostly made up of interleukin (IL) cytokines. These cytokines are produced by one type of leukocyte, a type of white blood cell, and activate another leukocyte.
The structure of both the cytokine and the cytokine receptor is highly specific, so only one kind of cytokine can bind to one cytokine receptor. This doesn’t mean that cytokine production and activation is as specific. Some cells produce different types of cytokines, and some target cells have more than one kind of cytokine receptor on their membrane so they can be stimulated by more than one kind of cytokine.
When a cytokine binds to its corresponding receptor, the receptor becomes activated. Usually, this means that a secondary messenger within the cell is stimulated by the cytokine receptor. The secondary messenger then causes the reactions that make the cell change its behavior. Common responses of a cell to cytokine stimulation include increasing or decreasing the expression of protein receptors on the cell membrane, secreting molecules, which could be other cytokines, and cell growth and multiplication.
Cytokine activity can be blocked by antagonist molecules. These molecules can decrease the efficiency of the cytokines in one of two ways. First, the antagonist can bind to the cytokine itself. This in turn will cause the cytokine to change its shape so it can no longer bind to the cytokine receptor. As the cytokine can no longer bind to the receptor molecule, the immune response is halted.
Secondly, antagonists can be a similar, almost identical, shape to the cytokine. When an antagonist is the same shape as a cytokine, then it can bind to the cytokine receptor. When an antagonist binds to the receptor, it then blocks the cytokine from binding there. Although the antagonist is bound to the receptor molecule on the cell membrane surface, it does not activate the receptor so the target cell is not stimulated either.