The Wheal An urticarial wheal results from release of histamine and other mediators from mast cells leading to an increase in capillary permeability with the escape of fluid from the circulation into the tissues.
Mast cells degranulate as a result of physical, chemical, pharmacological and immunological stimuli.
Ttype I hypersensitivity is mediated through the IgE attached to the mast cell which degranulates on exposure to the specific antigen. Mast cells can degranulate by other signals. Physical signals include vibration, heat, sunlight, and cold. In the biopsy of a typical urticarial lesion there is vasodilatation, dermal edema and perivascular infiltration of lymphocytes and eosinophils. Chronic urticaria may signal occult infections and other systemic diseases.