ALLERGIES/TYPE I HYPERSENSITIVITY

Allergies occur due to hypersensitivity of the immune system to substances in the environment that is normally harmless. There are four different kinds of hypersensitivity. Types I-III are antibody mediated. Antibodies, also called immunoglobulins, recognize antigens, foreign substances that induce an immune reaction in the body. Antigens that provoke an allergic reaction are called allergens. The five major immunoglobulin classes, or isotypes, found in serum are IgA, IgD, IgE, IgG, and IgM. Most allergies fall under the type I hypersensitivity category. This type of hypersensitivity manifests quickly, so it is also called an immediate hypersensitivity. It occurs thanks to IgE antibodies, so it is also called IgE-mediated hypersensitivity. IgE binds to the FcƐ receptor, which is found on mast cells and basophils. When an antigen binds to IgE specific to itself, it cross-links these receptors, activates the cell, and this induces release of chemical mediators from the mast cells which lead to allergic disease.