The physical barriers on the surface of the body play a critical 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 skin microbiota tends to exclude new invaders, while antimicrobial molecules in sweat 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 trap particles. The mucus also contains 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, complex commensal microbiota. If all else fails, invaders may be rapidly removed from the GI tract by vomiting and diarrhea.
The intestinal microbiota plays a key role in maintaining animal health. First, it is a source of nutrients, especially in herbivores, where it provides a means of exploiting a cellulose-rich diet and a source of essential vitamins. This microbiota also plays a critical role in the defense of the body. The large, well-adapted microbial population excludes many potential pathogens through competition. More importantly, the constant stimulus provided by the presence of these organisms stimulates the development of the adaptive immune system and regulates the intensity of inflammation mediated by the innate immune system.