Hypersensitivities

Hypersensitivity, or allergic reaction, occurs when the immune system overreacts to an allergen.

Hypersensitivity can be of four types.

Type I or immediate hypersensitivity, such as asthma or food allergies, manifests minutes after re-exposure to an allergen in previously sensitized individuals. Initially, exposure to specific allergens triggers substantial production of IgE antibodies, which adhere to mast cells and basophils. Subsequent encounters with the same allergen lead to its binding with pre-existing IgE antibodies, prompting these cells to discharge histamine and other inflammatory mediators. 

Type II, or cytotoxic hypersensitivity, such as a mismatched blood transfusion, is caused by antibodies binding to antigens on the blood or tissue cells. This initiates phagocytosis and induces cell lysis.

Type III, or immune-complex hypersensitivity, arises when antigen-antibody complexes deposit in tissues, causing inflammation and damage. Examples include rheumatoid arthritis and glomerulonephritis.

Lastly, type IV, or delayed hypersensitivity, usually appears 12–72 hours after exposure to an allergen. Upon reacting with antigens, T cells release gamma-interferon, leading to inflammation.