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 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 and 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 . 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.
In general, immune system disease takes three forms: insufficient immune function causing immunodeficiencies, Immunodeficiency Diseases in Animals Immunodeficiency diseases manifest clinically as a predisposition to infections. They are usually recognized when an animal makes multiple visits to a veterinarian for infections that would... read more manifested as increased susceptibility to infections; diseases resulting from excessive immune function Excessive Immune Function in Animals There are many ways by which excessive immune function can cause disease or death. These include excessive innate responses, excessive adaptive responses, autoimmune diseases, and tumors of... read more , resulting in inappropriate cell and tissue damage or hypersensitivities Hypersensitivity Diseases in Animals Excessive activity of the adaptive immune system can lead to inappropriate immune responses collectively called hypersensitivities. In general, inappropriate innate immune responses do this... read more ; and diseases in which the immune system directly attacks normal body cells and organs, called autoimmune diseases Organ-Specific Autoimmunity Excessive activity of the adaptive immune system can lead to inappropriate immune responses collectively called hypersensitivities. In general, inappropriate innate immune responses do this... read more . Some inflammation and limited tissue destruction are features of the normal innate and adaptive immune responses. Clinical disease occurs when this inflammation is excessive or in the wrong place or is directed against the wrong target.
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Also see pet health information regarding immunologic diseases in dogs Immune Disorders of Dogs and immunologic diseases in cats Immune Disorders of Cats .