The best Vitamin D to boost your immune system:

 The liver has to convert any type of Vitamin D to another form and then the kidneys add an oxygen molecule to it that makes it active. Most doctors
prescribe B2, a synthetic D that does little good and can be harmful.

D3 from animal cholesterol is closest to what sunlight produces in humans and is converted 500 times faster than D2. It’s estimated that D3 is 4 times more effective in humans than D2 and “vitamin D2 should no longer be regarded as a nutrient appropriate for supplementation or fortification of foods.”

No clinical trials have shown D2 to be effective at preventing bone fractures, for instance, while every trial of D3 does.

https://draxe.com/3-things-everyone-should-know-about-vitamin-d/ 
 

Additional info on Vitamin D:

http://articles.mercola.com/sites/articles/archive/2012/02/23/oral-vitamin-d-mistake.aspx