Cross-linking of IgE bound to FcεRI on tissue mast
cells or on circulating basophils triggers the release of granules
containing preformed mediators, including histamine, TNF-α,
proteoglycans, and neutral proteases, including tryptase, chymase,
and carboxypeptidase. It also results in the rapid de novo synthesis
of lipid-derived mediators, including prostaglandin D2,
the chief prostaglandin product of mast cells, as well as platelet-activating
factor and leukotrienes B4, C4, D4, and
E4. The release of these mediators induces an immediate
hypersensitivity reaction, which in an atopic individual is seen
as the “wheal and flare” skin reaction on scratching
of the skin with an allergenic substance. It is characterized by
vasodilatation, edema, and smooth muscle contraction. A similar
reaction pattern is seen in other tissues such as bronchial airways, where
mediator release on allergen inhalation rapidly induces mucosal
edema, mucus production, smooth muscle constriction, and reduced
airflow. The immediate hypersensitivity reaction is reversible and
usually subsides within 2 hours of its initiation.