some people develop an allergic reaction or hypersensitivity when exposed to substances such as dust pollens animal dander or penicillin this hypersensitivity is mediated by IgE sensitization occurs when the antigen makes contact with some part of the body the antigen is taken up processed by antigen presenting cells and presented on a class 2 MHC 2 T helper cells tissues under the mucous membranes are rich in b-cells committed to IgE production and IgE producing cells are more abundant in persons susceptible to allergies the T helper cells produce cytokines which stimulate these B cells to proliferate and differentiate into IgE producing plasma cells as IgE is produced in specific areas of the body the IgE molecules attached via their constant regions to receptors on nearby mast cells mast cells contain granules packed with chemicals that induce a hypersensitivity response once attached the IgE molecules can survive for many weeks the individual is now sensitized to the antigen when exposed to the antigen for a second time the antigen binds to the IgE antibodies on the mast cells to trigger a response to cell bound IgE molecules must react with a specific antigen within seconds of the reaction the mast cell releases histamine and other mediators of the inflammatory response from the granules triggering a variety of symptoms