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...