Conclusions and Future Directions

Vitamin A has been used as both disease-targeted and prophylactic therapy to reduce morbidity and mortality from infectious diseases for hundreds of years. Vitamin A plays an important role in haematopoiesis, the maintenance of mucosal surfaces, the function of T and B lymphocytes, NK cells and neu-trophils, and the generation of antibody responses to T-cell-dependent and independent antigens. As an immune modulator, vitamin A reduces the severity but not the incidence of certain types of infections: measles, diarrhoeal diseases, malaria and, possibly, infections related to pregnancy. Vitamin A does not appear to reduce the morbidity and mortality from ALRI. As a general rule, there appears to be little value in vitamin A supplementation in populations that are already relatively well-nourished and thus clinical investigation of immune modulation by vitamin A should be focused on populations at high risk of vitamin A deficiency. Despite the tremendous advances that have been made in our understanding of the role that vitamin A plays in immune function, many gaps in knowledge remain:

  • The relationship between vitamin A status in humans and the function of immune effector cells, such as neutrophils, macrophages, NK cells and cytotoxic T-cells.
  • The relationship between vitamin A status in humans and the balance between T-helper type 1-like and T-helper type 2-like immune responses.
  • The relationship between vitamin A status and gut integrity in humans.
  • The role of vitamin A in resistance to P. falciparum malaria.
  • The role of vitamin A in resistance to tuberculosis in humans.
  • The more precise biological mechanism(s) by which vitamin A reduces measles severity.
  • The more precise biological mechanism(s) by which vitamin A reduces diarrhoeal-disease severity.
  • The role of vitamin A in immune senescence.
  • The role of vitamin A in apoptosis.
  • The relationship between vitamin A and other micronutrients (e.g. zinc) in immune modulation.
  • The relationship between vitamin A status and expression of pro-inflammatory cytokines.
  • The relationship between vitamin A status and specific infections during pregnancy.
  • The uses of synthetic retinoids in immune modulation.

These are promising areas for future investigation, which should be addressed in order to gain further insight into the biological functions of this important vitamin.

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