Type III Hypersensitivity –Immune Complexes – 2-8h (Abbas pp 211)

Type III Hypersensitivity occurs as a result of deposition of antigen-antibody complexes – immune complexes – in tissue spaces. Usually these areas are in blood vessel sites where there is turbulence (branches of vessels) or high pressure areas. Thus, immune complex diseases are usually systemic (i.e.: vasculitis, arthritis, and nephritis). The deposition of immune complexes induces inflammation by activating leukocytes – similar to Type II Hypersensitivity.

• Antibodies in immune complexes (i.e: IgG & IgM) bind to Fc receptors of neutrophils and macrophages activating them ≫ inducing inflammation.

• IgM activates the complement cascade via the classical pathway, complement by-products induce further inflammation. Leukocytic activation results in enzymes released (“frustrated phagocytosis”) that cause damage to adjacent tissues.

• Leukocytes also release cytokines which increase blood vessel permeability ≫ increasing ability of immune complex to deposit in local tissues.

Note the process above is exactly the same as Type II, except the initiating factor is immune complex NOT antibodies.

If the reaction is localised it is termed an Arthus reaction (i.e.: deposition of immune complex is local), if it is systemic it is termed as serum sickness (i.e.: deposition occurs in blood vessels).

Arthus Reaction – Localised Type III Hypersensitivity Reaction (Abbas pp 263)

• This is an experimental immune complex mediated reaction. An Arthus reaction occurs when immune complexes deposit in local tissues ≫ local inflammatory response. IgG is already made, so injection of a foreign antigen subcutaneously means immune complexes will be formed subcutaneously locally. Because it is a low dose antigen – immune complexes are only formed locally  activating leukocytes and complement system. As a result inflammatory cells infiltrate the area (↑ vascular permeability), and blood flow is increased.

Serum sickness

• This is caused by injecting large doses of antigen into the blood stream. This causes immune complexes to be deposited within blood vessels. This occurs in high pressure and high turbulent areas (joints and kidneys). This causes leukocytic and complement activation causing inflammatory responses (i.e.: arthritis & vasculitis). Fever is characteristic. Symptoms resolve when foreign antigen is cleared.

• The large doses of antigen arise from horse serum because they contain antitoxin antibodies.

Category: Pathology Notes