The physical barriers on the surface of the body play a significant role in slowing or blocking microbial invasion. Very few microorganisms can penetrate intact skin; instead, invaders usually enter through wounds or by injections such as mosquito bites. Skin wounds heal rapidly to reestablish the protective barrier. A complex population of normal skin bacteria tends to exclude new invaders, while antimicrobial molecules in sweat can kill many would-be invaders. In the airways, the structure of the upper respiratory tract serves as an effective filter of small particles. The airways themselves are lined by a layer of adhesive mucus that can entrap microbes. The mucus contains multiple antimicrobial proteins such as defensins, lysozyme, and surfactants. “Dirty” mucus is constantly being replaced by clean material as ciliary action carries it to the pharynx where it is swallowed. Coughing and sneezing remove larger irritants from the airways and nasal passages and are essential defensive reactions. The defense of the intestine centers largely on the presence of the huge and immensely complex normal commensal flora. Potential invaders may be unable to colonize the intestine in the presence of a well-adapted population of commensal microbes. If all else fails, invaders may be rapidly removed from the GI tract by vomiting and diarrhea.
Last full review/revision July 2011 by Ian Tizard, BVMS, PhD, DACVM