IgE (Fc =ε) is the Ab responsible for hypersensitivity reactions.IgE complexes have a high affinity for host cell surfacesand can damage the host. High levels of IgE are foundin persons with allergies of various types, as well as in autoimmunediseases. The Fc fragment is responsible for theIg–cell reactivity. An Ab-plus-antigen reaction yields thetypical Ab–antigen complex. The Fc portion of the Ab is actuallypart of the mast cell. When antigen binds to the Fabportion of the Ab, the IgE molecules become cross-linked.This probably distorts the membrane of the mast cells andstimulates them to release histamine, which causesbronchial constriction, itching, redness, and anaphylaxis.
