The second category in Table 12.1 is compounds that are antioxidants or provide nutritional support. All cells need proper nutrition to function optimally, and immune cells are no exception. Although these require a variety of micro- and macronutrients, we focus here on a select few nutrients that have been extensively studied. In particular, several antioxidant vitamins appear to support immune function; when their levels are low, immune function can be hampered. Animal and human studies have reported that vitamins C and E support immune function through a number of mechanisms. For example, immune cells produce various noxious compounds, including reactive oxygen species (ROS) and reactive nitrogen species; antioxidants may help protect immune cells from self-destruction caused by the release of these compounds.
Antioxidants may also support immune function via their effects on intracellular glutathione levels. Glu-tathione plays an important role in immune function for three reasons. First, as a primary intracellular antioxidant, adequate glutathione is needed to protect the cell. Second, adequate glutathione levels are necessary for optimal T-cell activation by IL-2.17 Third, high glutathione levels inhibit cellular production of PGE2, an immunosuppressive substance.18 Glutamine, listed in the table, induces glutathione synthesis and in addition can support immune function by acting as a fuel for immune cells. Antioxidants can also increase glutathione concentrations.
Lastly, selenium can support immune function. Selenium is not an antioxidant itself, but it is a crucial component of the antioxidant enzyme glutathione peroxidase. It therefore assists in the antioxidant effects of glutathione (see Figure 5.2). Moreover, either because of its influence on glutathione or through other means, selenium can enhance the expression of IL-2 receptors on immune cells.19'20'21
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