A double-blind, placebo-controlled study from 2016 randomized elderly participants to receive 30 mg of zinc daily for 3 months. Serum zinc concentrations increased after supplementation, and researchers found that this increase led to an increased number of T-cells (specific cells that modulate immune responses), thereby enhancing T-cell function.
Barnett, J. B., Dao, M. C., Hamer, D. H., Kandel, R., Brandeis, G., Wu, D., Dallal, G. E., Jacques, P. F., Schreiber, R., Kong, E., & Meydani, S. N. (2016). Effect of zinc supplementation on serum zinc concentration and T cell proliferation in nursing home elderly: a randomized, double-blind, placebo-controlled trial. The American journal of clinical nutrition, 103(3), 942–951. https://doi.org/10.3945/ajcn.115.115188