Alright, did you read the entirety of the study that I linked over to you? I'll need to know what parts you're looking at and have questions on in particular to try to help. It goes over the immunomodulatory effects that vitamin D has, and how it's understood to be utilized within the immune response processes. So boosting the immune system likely just means making it more efficient, and providing it with adequate amounts of sufficient systemic vitamin D to use.

The extrarenal 1-α-hydroxylase enzyme in macrophages differs from the renal hydroxylase as it is not regulated by PTH[5]. Instead, it is dependent upon circulating levels of 25 D or it may be induced by cytokines such as IFN-γ, IL-1 or TNF-α[6]. Furthermore, the macrophage 24 hydroxylase enzyme is a non-functional splice variant, so there is no negative feedback of local 1,25 D production by 1,25 D.