Type III hypersensitivity also known as immune-complex allergic reaction is mainly characterized by an adverse inflammatory response mediated by antigen-antibody complexes. Antibodies react specifically with antigens to form small soluble immune-complexes which are finally cleared from the system by phagocytic cells, macrophages and the reticuloendothelial system (RES). The formation of immune-complexes in normal antigen-antibody reaction makes the invading pathogen easily opsonized and phagocytosed by phagocytes. However, in some antigen-antibody reaction the immune-complexes formed cannot be eliminated from the host’s body (due to the large size of immune-complexes formed); and these large insoluble immune-complexes (that usually comprises of IgM and IgG) find their way into various tissues of the body where they cause several tissue damages via the release of several proteolytic enzymes and other tissue-damaging substances.
IgM and IgG are the main effectors of Type III hypersensitivity reaction. The skin tissues, glomeruli (singular: glomerulus) of the kidney and the host’s blood vessels are some examples of the tissues that can be affected by the deposition of immune-complexes. Deposition of antigen-antibody complexes in the body’s tissues activates several components of the immune system especially the complement system; and the attempt by the polymorphonuclear (PMN) cells (e.g. neutrophils) to eliminate these immune-complexes result in inflammatory responses and tissue damage. Type III hypersensitivity reaction is mostly common amongst antigens or pathogens (e.g. viruses, bacteria and parasites or protozoa) whose pathogenicity in vivo is usually associated with some level of damage to their host tissues or cells. Immune-complex mediated allergic reactions i.e. Type III hypersensitivity could cause a localized reaction or systemic reaction depending on the route of entry of the causative antigen or allergen; and they generally occur when the immune system of the animal host becomes inundated by large insoluble immune-complexes.
Arthus reaction, a localized Type III hypersensitivity and serum sickness, a generalized or systemic Type III hypersensitivity are typical examples of immune-complex allergic reactions. Serum sickness is characterized by fever, arthritis, itching, oedema and rashes; and it occurs few days after large amounts of allergens are inoculated or injected into the bloodstream. In Arthus reaction, the allergen is injected intradermally or subcutaneously at high concentrations; and it is usually characterized by tissue necrosis and other inflammatory reactions at the site of administration. Type III hypersensitivity is also implicated in some clinical conditions including autoimmune diseases (e.g. rheumatoid arthritis), some microbial diseases (e.g. post-streptococcal glomerulonephritis), systemic lupus erythematosus and allergies to some drugs such as penicillin and sulphonamides.
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