Folic acid plays a crucial role in DNA and protein synthesis, suggesting that every mechanism in which cell proliferation intervenes may be altered. Cell-mediated immunity is especially affected by folate deficiency: the blastogenic response of T lymphocytes to certain mitogens is decreased in folate-deficient humans and animals, and the thymus is preferentially altered. The effects of folic acid deficiency upon humoral immunity have been more thoroughly investigated in animals than in humans, and the antibody responses to several antigens have been shown to decrease. Conversely, the phagocytic and bactericidal capacities of polymorphonuclear cells have been studied mainly in folate-deficient humans. However results in this field are controversial. Alterations in immune system functions could lead to decreased resistance to infections, as commonly observed in folate-deficient humans and animals.