disorders of the immune system

combined immunodeficiency

affects animals during early life, after serum levels of maternally derived antibodies have declined. Arabian foals with this disease often die from opportunistic infection that results from an absence or deficency of immunoglobulins.

Hypersensitivity type I (immediate hypersensitivity)

Type 1 hypersensitivity is an immediate hypersensitivity that occurs when chemical mediators from mast cells are released. Allergies (atopy) and anaphylactic shock (a severe reaction that may occur within seconds after an antigen enters the circulation) are type I hypersensitivity diseases. These disorders occur when immunoglobulin E (IgE) antibodies are formed in response to a previously encountered antigen. When the antigen is re-encountered, the IgE binds to receptors on mast cells, which results in the cross-linking of IgE and the release of mast cell mediators. Mast cell mediators cause smooth muscle contraction and increase the permeability of the vasculature within minutes. Mast cell mediators are also cytokines that attract cells of the inflammatory response (i.e., neutrophils and eosinophils) to the area. A variety of clinical signs are evident, depending on the location of the hypersensitivity reaction.

Mast cell derived mediators (vasoactive amines, lipid mediators, cytokines)

Cytokine-mediated inflammation (eosinophils, neutrophils)

Hypersensitivity type II (antibody-mediated diseases)

Antibody-mediated diseases in which the antibodies are directed against the animal's own cells or components of the extracellular matrix. Type II hypersensitivity disorders are mediated by IgG binding to receptors on the cell surfaces. IgM may also be involved, and the resulting immune complexes will serve to activate the complement system. The activation of complement leads to the initiation of the inflammatory response. Complement and Fc receptor-mediated recruitment and activation of leukocytes (neutrophils, macrophages)—> opsonization and phagocytosis of cells.—> abnormalities in cellular function (hormone receptor signaling)

Immune-mediated hemolytic anemia (IMHA)

a condition that causes the destruction of red blood cells by the host itself, and immune-mediated thrombocytopenia (IMT), which results in platelet destruction, are type II hypersensitivity disorders. In the case of IMHA, the antibodies bind to several surface receptors on the red blood cell membrane, where they undergo opsonization and are subsequently phagocytized. A similar mechanism occurs with IMT. Neonatal isoerythrolysis is an IMHA of neonates that occurs most often in foals and cats. The disorder results from the ingestion of colostrum that contains maternal antibodies against the fetal erythrocytes (Fig. 54.3). Transfusion reactions are also mediated by antibodies.

Hypersensitivity type III (immune complex-mediated diseases)

occurs when antibodies and antigens form complexes that are deposited in various blood vessels. Glomerulonephritis, which is caused by the deposition of antibody-antigen complexes in the kidney, is an example of type III hypersensitivity. Systemic lupus erythematosus is an immune-complex disorder characterized by the production of a large amount of autoantibodies to a diverse population of cells and tissues. its causes are not clear.

Hypersensitivity type IV (T cell-mediated diseases)

1. macrophage activation, cytokine-mediated inflammation

2. direct target cell lysis, cytokine- mediated inflammation

caused by the reaction of T lymphocytes against self-antigens in tissues. Contact hypersensitivity reactions, such as those that may occur in dogs after contact with plastic in food dishes and collars or in human beings after contact with poison ivy, cause tissue injury with a delayed response. The chemicals from these substances react with skin proteins, and the immune system recognizes this chemical-protein complex as foreign, thereby resulting in dermatitis. Type I diabetes, rheumatoid arthritis, and chronic infections such as tuberculosis are all T cell mediated auto immune diseases.

Lymphoma

a type of tumor characterized by the uncontrolled proliferation of lymphocytes, is another abnormality of the immune system. The immune system normally recognizes and destroys cancer cells before they become established in the body, but sometimes the cancer seems to become resistant and escapes the immune defense mechanisms.