Question

while allergies are an example of type I hypersensitivities, there exist other hypersensitivities as well. Briefly describe type II, III, and IV hypersensitivities, including the immune component involved and how they result in overreaction of the immune system.

Answer 1

• Type ll hypersensitivities :-

Type II hypersensitivity reaction is also called antibody mediated hypersensitivity reaction because this is the differentiating feature from other types of hypersensitivity.

Type II hypersensitivity reaction can occur in many parts of the body. Generally, they include :-lungs,skin,blood system,nerve,thyroid

Type II hypersensitivity reaction is characterised by antibodies directed toward antigens (substance that attracts the antibody to bind with) that are present on cell surfaces outside the cells.The antigens can either be from the body itself or from outside the body (for example, bacteria or microorganisms that infect the body).By the various biochemical mechanisms, the end result are tissue damage to the body.

• Type lll hypersensitivities :-

In type 3 hypersensitivity reactions, insoluble immune complexes (aggregations of antigens and IgG and IgM antibodies) form in the blood and are deposited in various tissues (typically the skin, kidney and joints).

This deposition of the antibodies may trigger an immune response according to the classical pathway of complement activation – for eliminating cells presenting foreign antigens (which are usually, but not in this case, pathogens). There are two stages relating to the development of the complexes, firstly the complex forms when IgG and IgM antibodies are bound to an antigen, after this, the complexes can form larger ones which can be cleared by the body. It is at the first stage of this formation where clearance is not possible and the antigen-antibody complex will spread. The reaction takes hours to days to develop.

Tissue damage results at the site of the immune complex with the influx of phagocytes and granuloctyes and the release of inflammatory mediators.

• Type lv hypersensitivities :-

Type 4 hypersensitivity reactions are often called delayed type as the reaction takes two to three days to develop. Unlike the other types, it is not antibody mediated but rather is a type of cell-mediated response.

CD8+ cytotoxic T cells and CD4+ helper T cells recognise antigen in a complex with either type 1 or type 2 major histocompatibility complex. The antigen-presenting cells in this case are macrophages which secrete IL-1, which stimulates the proliferation of further CD4+ T cell.

Re-exposure to the allergen results in a Th1 mediated response which stimulates the proliferation of the allergen-specific memory Th1 CD4+ T helper lymphocyte via recognition of complexes of peptide on antigen presenting cells (APCs).

CD4+ T cells secrete IL-2 and interferon gamma, further inducing the release of other Type 1 cytokines, thus mediating the immune response. Activated CD8+ T cells destroy target cells on contact while activated macrophages produce hydrolytic enzymes and, on presentation with certain intracellular pathogens, transform into multinucleated giant cells.