Classically, vitamin D is known primarily for its role in bone and muscle metabolism. More recently, there is a growing body of evidence documenting the importance of vitamin D in many other systems of the body, including the development and function of the immune system. One of the earliest studies reporting that alveolar macrophages can convert the storage form of vitamin D into the active form was done using porcine alveolar macrophages.4 It is also reported5 that antigen-presenting cells, such as dendritic cells, possess the enzyme 1α-hydroxylase, needed to convert storage 25-OHD into active 1,25(OH)2D. The same study5 reported that this process is up-regulated when stimulated by lipopolysaccharides, suggesting production of active vitamin D in this way may serve as a localized signal during infection to improve immune response. Another study6 showed activation of toll-like receptors (TLRs) on human macrophages increased expression of vitamin D receptors and the enzyme 1α-hydroxylase. Likewise, it has been shown7 that respiratory epithelial cells have the similar ability to up-regulate production of active vitamin D during viral infections. This ultimately leads to production of important antimicrobial peptides such as cathelicidin, as well as cluster of differentiation 14 (CD14; one of a group of cell-surface marker proteins), which are likely to be part of the host defense response. A notable review paper8 suggests a relationship between low levels of serum vitamin D and influenza epidemics in people. In a recent randomized placebo-controlled study,9 the authors noted a lower incidence of influenza virus infection in school children who received vitamin D supplementation.