Animals are under constant threat of microbial invasion. These potential invaders gain access to the body through the intestine and respiratory tract and the skin. The large and diverse microbiota of the intestine serves to protect the intestine from infectious invaders by occupying a niche that precludes other organisms from establishment there. Other potential invaders are infectious agents spread from other individuals.
To prevent microbial invasion, the body has as part of the innate immune system a series of defenses that collectively constitute a highly effective defense against invasion. These mechanisms include physical barriers Physical Barriers of an Animal's Body 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... read more such as the skin, which has its own microbiota and utilizes dessication as a mechanism to discourage colonization with other organisms. Inhaled microorganisms and other material are rapidly removed by the mucociliary apparatus, which consists of 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 the cough reflex.
The second line of defense is a “hard-wired” system of innate immunity Innate Immunity in Animals Acute inflammation is the central feature of innate immunity. The first step in the inflammatory process is the early detection of either invading organisms or damaged tissues. Most invaders... read more 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 illness responses such as a fever.
The third line of defense is the highly complex, specific, and long-lasting adaptive immunity Adaptive Immunity in Animals Innate immune responses, although critical to the defense of the body, cannot guarantee protection. They lack the flexibility to respond optimally to a diverse set of microorganisms and they... read more . Because an animal accumulates memory cells after exposure to pathogens, adaptive immunity provides an opportunity for the host to respond to exposure by creating a highly specific and effective response to each individual infectious agent. In the absence of a functional adaptive immune system survival is unlikely.
Also see pet health content regarding the biology of the immune system in dogs The Immune System of Dogs The immune system consists of a network of white blood cells, antibodies, and other substances that fight off infections and reject foreign proteins (see Table: Specialized Cells and Molecules... read more , cats The Immune System of Cats The immune system consists of a network of white blood cells, antibodies, and other substances that fight off infections and reject foreign proteins. In addition, the immune system includes... read more , and horses The Immune System of Horses The immune system consists of a network of white blood cells, antibodies, and other substances that fight off infections and reject foreign proteins. In addition, the immune system includes... read more .