Type I, or immediate hypersensitivity, encompasses IgE-mediated responses to foreign antigens and are collectively called allergic diseases. These reactions may be minor and local or may be severe and generalized. In its most extreme form, type 1 hypersensitivity or allergy is expressed as a lethal shock syndrome called anaphylaxis. Anaphylaxis is an acute systemic manifestation of the interaction of an antigen (allergen) binding to IgE on mast cells and basophils. The antigen binding triggers the mast cell to release a complex mixture of inflammatory mediators, including the following:

Type II hypersensitivity reactions result when antibodies attack normal cells. These bound antibodies can then activate the classical complement pathway, resulting in cell lysis, or phagocytosis. One source of these antibodies is cross-reactions. Cross-reactive antibodies can develop during infections. These cross-reactive antibodies directed toward an infectious agent may also bind to normal tissue antigens and trigger antibody-mediated cytotoxicity.

A type III hypersensitivity reaction is the deposition of antigen-antibody complexes (immune complexes) in tissues or small blood vessels, which activate complement in situ and cause acute inflammation. This activation results in the production of peptides (complements 3a and 5a [C3a and C5a]) that are chemotactic for neutrophils.

Canine rheumatoid arthritis is rare in dogs and results from excessive, uncontrolled inflammation around joints that causes joint and bone erosion. It appears to be mediated by immune complex deposition in the synovia that results in the development of a nonsuppurative, proliferative synovitis. This is followed by joint erosion and eventually leads to fibrous ankylosis.

A nonerosive polyarthritis of unknown etiology is most common in large dogs, particularly German Shepherd Dogs, Doberman Pinschers, retrievers, spaniels, and pointers. In toy breeds, it is most frequent in Toy Poodles, Yorkshire Terriers, and Chihuahuas, or mixes of these breeds. There is no evidence of a primary chronic infectious disease process or of systemic lupus erythematosus. Arthritis is often the sole manifestation. It is commonly associated with the simultaneous presence of respiratory or GI disease and some neoplasia.

A type III hypersensitivity reaction is the deposition of antigen-antibody complexes (immune complexes) in tissues or small blood vessels, which activate complement in situ and cause acute inflammation. This activation results in the production of peptides (complements 3a and 5a [C3a and C5a]) that are chemotactic for neutrophils.

Type IV hypersensitivity reactions, or cell-mediated inflammatory reactions, occur when antigen triggers local cell-mediated responses. Multiple cytokines are produced, as well as activated macrophages and cytotoxic T cells. The infiltration of mononuclear cells and the release of inflammatory molecules by these cells result in intense local inflammation.

Along with immunodeficiencies and hypersensitivities, the third type of immunologically mediated disease involves the development of autoimmunity. Autoimmune diseases result when an immune response is directed against normal cellular components.

Systemic lupus erythematosus (SLE) is a complex autoimmune disease that occurs in dogs. SLE is rare in cats and in large animals. It has two consistent immunologic features: disseminated immune complex disease and a tendency to produce multiple autoantibodies. Clinically, it reflects a combination of type II and III hypersensitivities.

Deutskens F, Lamp B, Riedel CM, et al. Vaccine-induced antibodies linked to bovine neonatal pancytopenia (BNP) recognize cattle major histocompatibility complex class I (MHC I). Vet Res. 2011;42(1):97. doi:10.1186/1297-9716-42-97

Also see pet health information regarding disorders involving anaphylactic reactions in dogs Disorders Involving Anaphylactic Reactions (Type I Reactions, Atopy) in Dogs and anaphylactic reactions in cats Disorders Involving Anaphylactic Reactions (Type I Reactions, Atopy) in Cats .