Vitamin D is a steroid hormone, that is stimulated into production from the UVB of sunlight. I need to check which. Note that overexposure to sunlight ends up destroying the vitamin D in the skin. This destructiong is a self-preservation mechanism that prevents the body from producing too much vitamin D.
There seems to be a strong connection between Vitamin D to proper inflammatory response and the immune system.
Cholesterol is vital to Vitamin D production; Cholesterol is a precursor to Vitamin D.
Vitamin D3 (Cholecalciferol) is produced from sunlight exposure on the skin. Note that all by itself, Vitamin D3 is not biologically active. It needs to be converted to 25 hydroxy Vit. D3 in the liver, then 1,25 dihydroxy Vit. D3 in the kidneys.