Animals are under constant threat of invasion by a diverse range of microorganisms that seek to enter the body and exploit its resources for shelter and food. To ensure survival and prevent such exploitation, the body combats the most dangerous of these invaders with an equally complex set of defensive mechanisms that can be thought of as a series of barriers. These mechanisms include physical barriers to invasion such as a tough, thick skin or the ability to cough and sneeze. The second line of defense is a “hard wired” system of innate immunity that depends on a rapid, stereotyped response to stop and kill both bacterial and viral invaders. This is typified by the process of acute inflammation and by the classic sickness responses such as a fever. The third line of defense is the highly complex, adaptable, and incredibly effective adaptive immune system.
Innate immune responses are highly effective against opportunistic organisms or those of low virulence, but by their very nature cannot do more than delay highly pathogenic microbial invaders. Longterm resistance and survival depends on adaptive immunity. The adaptive immune system is effective against a wide variety of pathogens. Its effectiveness improves each time it is activated in response to microbial invasion. Because the body accumulates immune memory cells as it ages, adaptive immunity provides an almost insurmountable barrier to most potential invaders. In its absence, animals die.
The adaptive immune system faces complex challenges. Many different microorganisms, including bacteria, viruses, protozoa, and helminths, may attempt invasion. The optimal immune responses to this diversity of invaders must also be very diverse. For example, invaders such as bacteria that live outside body cells are best attacked by an antibody-mediated (or humoral) immune response, whereas viruses living within cells are best destroyed by the killing of infected cells through cell-mediated mechanisms.
Last full review/revision August 2013 by Ian Tizard, BVMS, PhD, DACVM