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From a therapeutic point of view, immunomodulation refers to any process in which an immune response is altered to a desired level. Microorganisms are also capable of modulating the response of the immune system to their presence, in order to establish or consolidate an infection. Thus, immunomodulation can be beneficial or detrimental to a host.

Many providers of nutritional supplements claim that a product enhances certain aspects of the immune system so as to more vigorously shield the body from infection or the development of maladies such as cancer. However, rigorous testing of these claims is typically lacking and so the claims of nutritional links with immune system improvement are at present tenuous.

A firmer link exists between exercise and immunomodulation. Moderately active people are known to have macrophages that are more capable of killing tumors, due to the increased production of a compound called nitric oxide. This population also displays lower incidence rates for cancer and other chronic diseases. Even sporadic exercise increases the ability of an immune system component called natural killer cells to eradicate tumors.

Conversely, too much exercise is associated with increased susceptibility to respiratory tract infections, indicating that the immune system is impaired in the ability to thwart infections.

Immunomodulation by microorganisms is directed at several aspects of the immune system. One target are the small molecules known as cytokines , which function as messengers of the immune system. In other words, cytokines stimulate various immune responses such as inflammation of the manufacture of antibodies. Other cytokines are involved in down-regulating the immune responses.

Some microorganisms are able to produce and excrete proteins that mimic the structure and function of cytokines. Often the result is a suppression of the host's inflammatory response. Examples of microbes that produce cytokine-like molecules are the Epstein-Barr virus , poxvirus, vaccinia virus.

Other microbes, such as the protozoan Trypanosoma cruzi blocks the activation of cytokines by an as yet unknown mechanism. The result is a severe suppression of the immune system. Adenoviruses also block cytokine expression, at the level of transcription .

The manipulation of cytokine expression and action may also be exploited to produce vaccines. For example, vaccines designed to nullify or enhance the activity of certain cytokines could cause greater activity of certain components of the immune system. While vaccines have yet to achieve this level of activity, specific experimental targeting of deoxyribonucleic acid has suppressed certain cytokines.

Another portion of the immune system capable of immunomodulation is complement . Herpes simplex virus types 1 and 2, the viruses responsible for cold sores and genital herpes in humans, resist the action of complement. The presence of specific viral proteins are required, and may act by disrupting a key enzyme necessary for complement manufacture.

Vaccinia virus can also evade complement action, via a protein that structurally resembles a host protein to which complement binds. Also, another viral protein, called the inflammation modulatory protein, acts to decrease the inflammatory response at the site of infection, thus preserving host tissue from damage and providing the virus particles with relatively undamaged cells in which to grow.

The protozoan Trypanosoma cruzi can regulate the activity of complement before infecting human cells. Once inside the cells of the host, the parasite can evade an immune response.

A variety of bacteria , viruses, and parasites are also able to modulate the immune system by affecting the way antigens are exposed on their surfaces. Antigen presentation is a complex series of steps. By controlling or modulating even one of these steps, the antigen presentation process can be disrupted. The formation of antibody is thus affected.

Aside from biological agents, physiological factors can cause immunomodulation. For example, stress is known to be capable of suppressing various aspects of the cellular immune response. The release of various hormones may disrupt in the normal expression of cytokines. Specifically, those cytokines that suppress inflammation are more evident, either because of their increased production or the decreased production of cytokines that activate inflammation.

See also Immunologic therapies