Microbes living in mammalian guts have an important role in intestinal immunity and it seems that those living on the skin are similarly crucial for tuning immune responses to skin pathogens.
Yasmine Belkaid at the US National Institute of Allergy and Infectious Diseases in Bethesda, Maryland, and her colleagues compared mice with microbes on their skin with germ-free mice raised in aseptic conditions. T cells, a subset of immune cells, produced fewer immune-stimulating molecules in germ-free animals than in control mice. When infected with a skin parasite, the germ-free mice developed a greater number of parasites per skin lesion than the controls, and also showed impaired T-cell responses. Populating the skin with a skin bacterium restored immunity to the germ-free animals.
Science http://dx.doi.org/10.1126/science.1225152 (2012)
For a longer story on this research, see go.nature.com/8ahyc3