Veterinary Immunology: Types and Mechanisms of Hypersensitivity

Hypersensitivities

Aberrations of normal host defense processes; immune responses that are normally protective can cause damage when they are excessive.

Type I

IgE mediator; examples include insect bite reaction, hay fever, asthma late phase reaction, drug reaction, and food allergies.

Type II

IgG, IgM cytotoxic or complement mediated; examples include transfusion reaction, neonatal isoerythrolysis, autoimmune hemolytic anemia and thrombocytopenia, and hemolytic reaction to drugs.

Type III

IgG, IgM immune complex mediated; examples include immune complex glomerulonephritis, purpura hemorrhagica, staphylococcal dermatitis, and systemic lupus erythematosus.

Type IV

Sensitized T cells delayed hypersensitivity; examples include allergic contact dermatitis and tuberculin reaction.

Atopy

Tendency to develop allergic diseases; genetically determined disorder with an increased capacity to form antibodies and to acquire certain allergic diseases.

Complement-mediated lysis

Occurs when the antibody reacts with the antigen determinant on the surface of the target cell, with complement fixation leading to the lysis of the target cell.

Antibody-dependent cellular toxicity

Antibody attaches to the target antigen by its antibody-combining site; Fc end of the antibody is free to interact with cytotoxic cells (macrophages, neutrophils, NK cells), bringing them in close proximity with the target cell and facilitating cell destruction.

Immune-Complex Disease

Produced by the combination of antibody and antigen resulting in the formation of immune (toxic) complex; this toxic complex, located in tissues, attracts leukocytes by the chemotactic effect of complement activation leading to mediator activation producing a localized inflammatory reaction that ultimately leads to tissue injury.

In situ formation

Where antibodies leave the circulation to bind to antigens in certain tissue sites like in the case of immune-complex glomerulonephritis.

In circulation formation

Where immune complexes accumulate in filtering structures such as the glomeruli, synovia, and choroid plexus. The antibody is not directed against the host tissue and the site of injury is dependent on where the complexes lodged.

Arthus reaction

Example of localized complex induced lesion where the reaction took place in a vessel wall.

Arthus reaction mechanism

Antigen is injected into the skin and the circulating antibody migrates and meets the antigen in the vessel wall.

Arthus reaction response

Complement is activated and an acute inflammatory response follows with the predominance of neutrophils.

Arthus reaction timing

Maximum reaction usually occurs within 3-6 hours.

Serum sickness

Follows after the intravascular administration of large amounts of foreign protein.

Serum sickness mechanism

Much of the antigen is removed by protein catabolism, complexes with developing antibody will form.

Serum sickness deposition

Large complexes are phagocytized but there can be a widespread complex deposition in places like the kidney, joints and blood vessels.

Serum sickness occurrence

Condition is now rare but occurred commonly when animals such as horses are used for antiserum production.

Type IV (Delayed Type)

A classification of immune response.

Allergic contact dermatitis

Occurs in dogs, horses and rarely in cats.

Hapten

A small molecule that, when bound to a protein in the skin, initiates an immune reaction.

Sensitized T cells

T cells that have been activated by exposure to an antigen and migrate back to the skin to interact with that antigen.

Cytokines

Signaling molecules released by T cells to recruit other inflammatory cells, particularly macrophages.

Fleabite dermatitis

A condition in dogs characterized by self-trauma, secondary infection, and a type I hypersensitivity reaction.

Inflammatory cells

Cells involved in the immune response, including eosinophils, plasma cells, and neutrophils.

Tuberculin reaction

A diagnostic test prototype of Type IV hypersensitivity, where intradermal injection of tuberculin causes swelling in sensitized individuals.

Mycobacterium tuberculosis

The bacterium that causes tuberculosis, which is used in the tuberculin skin test.

Chronic granulomatous inflammation

An inflammation involving persistent antigens, such as Mycobacterium, that resist phagocytosis.

Cytotoxic T cells

T cells believed to be involved in tissue injury during chronic granulomatous inflammation.

Macrophages

Immune cells that are attracted and activated by cytokines to remain at the reaction site during chronic inflammation.

Granuloma

A structure formed during chronic inflammation, characterized by a collection of macrophages that may show epithelioid and giant cell formation.

Edema

Swelling caused by the accumulation of fluid in tissues, observed in the tuberculin reaction.

Mononuclear cells

A type of immune cell that infiltrates the site of a tuberculin reaction.

Lymphokines

A subset of cytokines released by lymphocytes that play a role in the immune response.

Intradermal injection

A method of administering a substance into the dermis, used in the tuberculin test.

Type I reaction

An immediate hypersensitivity reaction, such as that seen in fleabite dermatitis.

Type IV reaction

A delayed-type hypersensitivity reaction, exemplified by the tuberculin reaction.