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Vitamin D Influence on Immune Response
Vitamin D is known as an immune regulator that assists in the adaptive and innate immune response. A deficiency in Vitamin D, from hereditary or environmental influence, can lead to a more inefficient and weaker immune response and seen as a contributing factor to the development of autoimmune diseases. With Vitamin D present, vitamin D response elements (VDRE) are encoded and expressed via pattern recognition receptors (PRR) responses and the genes associated with those responses. The specific DNA target sequence expressed is known as 1,25-(OH)2D3. The expression of 1,25-(OH)2D3 can be induced by Macrophages, Dendritic cells, T-cells, and B-cells. In the presence of 1,25-(OH)2D3, the immune system's production of inflammatory cytokines are suppressed and more tolerogenic regulatory T-cells are expressed. This is due to Vitamin D's influence on cell maturation, specifically T-cells, and their phenotype expression. Lack of 1,25-(OH)2D3 expression can lead to less tolerant regulatory T-cells, larger presentation of antigens to less tolerant T-cells, and increased inflammatory response.

Source
Lucas R, Rodney Harris R (2018) On the nature of evidence and ‘proving’ causality: smoking and lung cancer vs. sun exposure, vitamin D and multiple sclerosis. Int J Environ Res Public Health 15:1726

Harrison, S.R., Li, D., Jeffery, L.E. et al. Vitamin D, Autoimmune Disease and Rheumatoid Arthritis. Calcif Tissue Int 106, 58–75 (2020). https://doi.org/10.1007/s00223-019-00577-2