Immune System - Merck Veterinary Manual

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The Biology of the Immune System
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.
Immunologic Diseases
The function of the immune system is the detection and destruction of invading microorganisms and abnormal cells. Because of the great diversity of microbial invaders, the immune system is a complex mixture of protective mechanisms. These may be simply classified as innate immunity and adaptive immunity. Protection within the first few hours and days of microbial invasion is the responsibility of the “hard wired” innate immune system. Longer-term protection is the responsibility of the adaptive immune system.