The primary role of the immune system is the detection and destruction of invading microorganisms. Because of the great diversity of microbial invaders, the immune system has evolved an equally complex mixture of protective mechanisms. These may be simply classified as innate immunity (see The Biology of the Immune System: Innate Immunity) and acquired immunity (see The Biology of the Immune System: Acquired Immunity). Protection within the first few days of microbial invasion is the responsibility of the “hard wired” innate immune system. Longterm protection is the responsibility of the acquired immune system.

In general, disease associated with the immune system takes two forms: insufficient immune function causing immunodeficiencies, manifested as increased susceptibility to infections, and diseases resulting from excessive immune function, resulting in hypersensitivities and autoimmunity.

Under certain circumstances, normally protective immune responses can cause significant tissue damage. In general, excessive innate immune responses do this by triggering inappropriate inflammation leading to collateral damage to nearby tissues, or by producing vastly excessive amounts of inflammatory cytokines. Excessive acquired immune responses, in contrast, can cause damage by multiple mechanisms. One simple classification divides diseases due to excessive acquired immune responses into four distinct types. Three of these types are mediated by antibodies (Types I, II, and III), while Type IV is T cell-mediated.

Inflammation and limited tissue destruction are features of the normal innate and acquired immune responses. Clinical disease occurs when this inflammation is excessive or in an inappropriate location. This may be due to external environmental factors, such as the composition of the intestinal microflora, together with genetic and hormonal influences.

Last full review/revision July 2011 by Ian Tizard, BVMS, PhD, DACVM