Physical Barriers of the Body
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 being injected, such as by 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 with 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 microbiota. 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.
The intestinal microbiota plays a critical role in maintaining animal health. First, it is a source of nutrients, especially in herbivores, in which 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 the microbiota effectively stimulates the correct development of the adaptive immune system and regulates the level of inflammation mediated by innate immune systems.