What's Immediate hypersensitivity, ( type I hypersensitivity)?

A type of pathologic reaction that is caused by the release of mediators from mast cells. This reaction most often depends on the production of immunoglobulin E (IgE) antibody against environmental antigens and the binding of IgE to mast cells in various tissues. (Most frequent of all hypersensitivity,known as allergy or atopy )