Animal and human studies suggest that vitamin B6 deficiency affects both humoral and cell-mediated immune responses. Lymphocyte differentiation and maturation are altered by deficiency, delayed-type hypersensitivity responses are reduced, and antibody production may be indirectly impaired. Although repletion of the vitamin restores these functions, megadoses do not produce benefits beyond those observed with moderate supplementation. Additional human studies indicate that vitamin B6 status may influence tumor growth and disease processes. Deficiency of the vitamin has been associated with immunological changes observed in the elderly, persons infected with human immunodeficiency virus (HIV), and those with uremia or rheumatoid arthritis. Future research efforts should focus on establishing the mechanism underlying the effects of vitamin B6 on immunity and should attempt to establish safe intake levels that optimize immune response.