Animals are under constant threat of microbial invasion. These invaders usually gain access to the body via the intestine, respiratory tract, and skin.
The large and diverse microbiota of the intestine serves to protect it from infectious invaders by occupying a niche that precludes other organisms from establishment there. Other potential invaders include infectious agents transmitted by other animals.
To prevent this microbial invasion, the body uses multiple strategies that collectively constitute a highly effective defense.
The first line of defense comprises physical barriers such as the skin, which has its own microbiota and uses desiccation as a mechanism to discourage colonization with other organisms. Inhaled microorganisms and other material are rapidly removed by the ciliated epithelial cells and mucus-secreting cells that move inhaled material from the lower to the upper respiratory tract, from which they are removed by coughing or swallowing.
The second line of defense is a “hard-wired” system of innate immunity that depends on a rapid stereotypical response to stop and kill both bacteria and viruses. This is typified by the process of acute inflammation and by the classic sickness responses, such as fever.
The third line of defense is the highly complex, specific, and long-lasting adaptive immune system. Because an animal accumulates memory cells after exposure to pathogens, adaptive immunity provides an opportunity for the host to respond to exposure by mounting a highly specific and effective response to each individual infectious agent. In the absence of a functional adaptive immune system, survival is unlikely.
