I found a very informative research paper ~ on the National Institute of Health,
Vitamin D and the Immune System
Just in case the link above does not work:
The article is protected by Copyright. I can link to article.
I can not pull quotes from article.
Vit. D plays a far more important role in the Immune System and in
the treatment of autoimmune disease than originally believed.
Being Vit. D. deficient ~ also interferes with Sleep.