The American Journal of Clinical Nutrition 1991; 53:90-4 “Effect of B-Carotene on Lymphocyte Subpopulations in The Elderly Humans: Evidence For a Dose-Response Relationship” In elderly subjects (mean age 56), supplementing for two months with beta carotene led to immune improvement (an increase in the percentage of lymphoid cells with surface markers for T-helper cells, cells with interleukin II), and natural killer cells). Retinol (vitamin A) did not produce this effect. A dose of 180 mg/day of beta carotene for two weeks increased the total number of T-helper cells, and had no effect on suppresser cells.
Am J Clin Nutr40:1090-95, 1984. Research showed that deficiency of vitamin A is associated with an increased susceptibility to infection.
Aust Paediatr J 22(2):95-99, 1986 Preschool children with a history of frequent respiratory illness were given either 1500 IU vitamin A, or a placebo. After 11 months, the children given the vitamin A had nearly 1/5 fewer episodes of respiratory symptoms compared to the children who were not supplemented. Interestingly, the plasma retinol levels did not change. Children with prior histories of allergies or of lower respiratory illness benefited most.