How does Vitamin-D boost up the immune system:

Nearly all immune cells have vitamin D receptors, showing vitamin D

interacts with the immune system.

The active vitamin D hormone, calcitriol, helps regulate both the

innate and adaptive immune systems, our first and second lines of defence against pathogens.

And vitamin D deficiency is associated with immune dysregulation, a

breakdown or change in the control of immune system processes.

Many of the ways calcitriol affects the immune system are directly

relevant to our ability to defend against viruses.

For example, calcitriol triggers the production of cathelicidin and

other defensins – natural antivirals capable of preventing the virus

from replicating and entering a cell.

Calcitriol can also increase the number of a particular type of immune

cell (CD8+ T cells), which play a critical role in clearing acute viral

infections (such as influenza) in the lungs.

Calcitriol also suppresses pro-inflammatory cytokines, molecules

secreted from immune cells which, as their name suggests, promote inflammation. 

Some scientists have suggested vitamin D might help to alleviate the "cytokine storm” described in the most severe COVID-19

cases.