Whether vitamin D protects the body from infection is controversial. Researchers in California provide a possible explanation for why some studies have failed to link vitamin D to immune response: the vitamin does not work alone to boost immunity.

Richard Gallo at the University of California, San Diego, in La Jolla, and his colleagues found that, in human skin cells, one of two hormones — parathyroid hormone (PTH) and a related peptide, PTHrP — needs to team up with vitamin D's active form to increase the production of the antimicrobial peptide, cathelicidin. Mice injected with PTH fended off a Streptococcus infection more successfully than control animals, but only if they could produce the active form of vitamin D. The hormones can compensate for insufficient vitamin D and may participate in a positive feedback system with vitamin D and cathelicidin.

Sci. Transl. Med. 4, 135ra66 (2012)