Cytokine signaling is an important part of the human body regulation. Most cytokines are cell-secreted proteins from glial cells in the nervous system and are necessary for intracellular signaling. Most cytokines are local regulators that alert and activate lymphocytes. Some cytokine-signaling pathways involve hormones such as growth hormones and leptin, the hormone that controls fat storage.
The immune system depends on cytokine signaling to keep the human body healthy. Macrophages and dendritic cells engulf foreign particles and send a cytokine signal to nearby dormant lymphocytes. The receptors on the lymphocytes recognize the signal and activate. Those cells are specialized to recognize certain antigens. The combination of the macrophages and activation of lymphocytes through cytokine signaling help keep the body in homeostasis — or the proper internal equilibrium.
Some cytokine signals are not local but rather travel a long distance throughout the body. These cytokines are sometimes classified as hormones. This classification is changing, however, because cytokines are not secreted from glands. Instead, they are secreted from glial cells of the nervous system. These growth hormones are essential for embryonic development.
Cytokines bind to receptors on target cells and activate a cascade of intercellular signals. The most common of these pathways is the protein kinase transduction cascade. After the cytokine binds to the receptor embedded in the membrane of the cell, inactive protein kinases are activated by a process known as phosphorylation.
The phosphate is removed from an adenosine triphosphate (ATP) molecule and is attached to the inactive protein kinase. This now-active protein kinase phosphorylates a different inactive protein kinase molecule. The cascade continues, amplifying the signal as it goes. Eventually, the signal reaches a protein that creates a cellular response.
Another intercellular response that can be activated by cytokine signaling is a G-protein signaling pathway. The cytokine attaches to the G-protein-coupled receptor on the outside of the cell, and a guanosine diphosphate (GDP) molecule is phosphorylated. This activates an enzyme that controls the cellular response.
Cytokine signaling can be inhibited. Competitive inhibitors can bind to the receptor on the cytokine's target cell. Most cytokine signaling suppression is a result of feedback inhibition. When the product of the pathway becomes overwhelming, it will block the binding of the cytokine to the receptor. This shuts down the pathway, and no more product is created.