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The immune system is highly complex, and it includes an enormous variety of cells. Some are general all-purpose cells which can potentially destroy several different types of pathogens, while others are highly specific, taking on only certain types of targets.
To make this system operate effectively requires that these different cell types work in cooperation with one another, and while some cells communicate directly, with physical contact, others rely on chemical signals from proteins called cytokines.
There are four main categories of cytokines: interleukins, chemokines interferons, and growth factors. Within these categories are many different types of proteins, each with its own role in the immune system. However, there is also a large amount of redundancy in the cytokine system, and many of these chemical signaling molecules have overlapping functions.
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Currently there are 35 known interleukins, each with its own number designation, from IL-1 to IL-35. Functions of various cytokines include:
· Stimulation of immune cells (such as T or B cells)
· Stimulation of the inflammatory response
· Differentiation and maturation of lymphocytes
· Potentiation of antibodies (making their immune activity more potent)
· Inhibition or stimulation of the production of other cytokines
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Chemokines are small molecules which act as chemotaxic signals: they tell cells where in the body they need to travel to, essentially providing the cell with a sort of homing signal. Many are involved in the immune system, while various different chemokines have similar functions in other bodily systems.
When a chemokine is produced and released by cells, any cell which has a receptor for that chemokine can follow the chemical gradient to its source, thus reaching the location where it can carry out its function.
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This class of cytokine is produced usually in response to intracellular pathogens, including viruses and parasites. Interferons are also involved in the anti-tumor response (and as such their possible use in cancer therapy has been widely investigated).
Interferons are produced by cells in response to a unique feature of viral organisms: the presence of double-stranded RNA. These cytokines, once activated, in turn activate natural killer cells and macrophages, two immune cells which are effective at killing cells which have been infected by viruses.
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Growth factors are a class of cytokine which stimulates the proliferation of cells. As with many other cytokines, some growth factors are not specifically immune molecules, and can play other roles in the body at various times. Many growth factors are involved with differentiation of immune cells within the bone marrow.