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 Innate Immunity in Animals Innate immunity is one of two main immunologic strategies found in animals, the other being adaptive immunity. Major functions of the innate immune subsystem include anatomic defenses against... 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 sickness responses, such as fever.
The third line of defense is the highly complex, specific, and long-lasting adaptive immune system 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 mounting a highly specific and effective response to each individual infectious agent. In the absence of a functional adaptive immune system, survival is unlikely.
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Also see pet health content regarding 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 .