Some of the permanent microbial flora inhabiting the skin protects the host from other pathogens. The complex host– microbe and microbe–microbe interactions that exist on the surface of human skin illustrate that the microbiota have a beneficial role, much like that of the gut microflora. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2746716/
It is apparent that just as host immunological factors and behaviors shape the composition of these communities, microbes present on the skin greatly impact the functions of human immunity. Thus, today the skin immune system should be considered a collective mixture of elements from the host and microbes acting in a mutualistic relationship. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4219649/
Eccrine sweat glands secrete water to the surface of the skin. The water also contains salt and electrolytes which work to acidify the skin. This makes the skin a cool, dry, and slightly acidic barrier.
Furthermore, eccrine sweat glands constitutively express several antimicrobial peptides (AMPs), including cathelicidin and β-defensins. - https://www.ncbi.nlm.nih.gov/pubmed/16374474
Together, these factors are the reason why a desk would be more hospitable for some microbes