Vitamin B6, although having no anti-oxidant properties, plays an important part in anti-oxidant defenses because of its action in the metabolic pathway for the formation of cysteine, which, as indicated earlier, is the rate limiting precursor in glutathione synthesis. Vitamin B6 status has widespread effects on immune function (79). Vitamin B6 deficiency causes thymic atrophy and lymphocyte depletion in lymph nodes and spleen. Antigen processing is unaffected. However, the ability to make antibodies to sheep red blood cells is depressed. In human studies, the ability to make antibodies to tetanus and typhoid antigens is not seriously affected. Various aspects of cell-mediated immunity are also influenced by vitamin B6 deficiency. Skin grafts in rats and mice survive longer during deficiency, and guinea-pigs exhibit decreased delayed hypersensitivity reactions to bacille Calmete-Guerin (BCG) administration. Deficiency of vitamin B6 is rare in humans but can be precipitated with the anti-TB drug isoniazid. However, experimental deficiency in elderly subjects has been shown to reduce total blood lymphocyte numbers and decrease the proliferative response of lymphocytes to mitogens (80). Similarly, IL-2 production is reduced by deficiency of the vitamin. Restoration of vitamin B6 intake to normal by dietary supplements restores immune function. It is unclear, at present, whether a similar situation occurs in younger subjects.
One mechanism for the effect of vitamin B6 on immune function may be due to the importance of the vitamin in cysteine synthesis, as outlined earlier. Deficiency of the vitamin may limit the availability of cysteine for glutathione synthesis. In rats, vitamin B6 deficiency resulted in decreases of 12% and 21% in glutathione concentrations in plasma and spleen, respectively (81). In healthy young women, large doses of vitamin B6 (27mg/day for 2 weeks) resulted in a 50% increase in plasma cysteine content (82), presumably by increased flux through the transulfuration pathway. As cysteine is a rate-limiting substrate for glutathione synthesis, these findings may have implications for the response to pathogens because of the importance of glutathione in lymphocyte proliferation and anti-oxidant defense. However, although vitamin B6 has cellular effects on the immune system, evidence is lacking of any effect on the inflammatory response.
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