Immune-mediated mast cell activation includes Type I hypersensitivity reactions, in which allergen-bound IgE antibodies bind to high-affinity cell surface receptors on mast cells and basophils Autoimmune disorders, in which antibodies to an IgE receptor functionally cross-link IgE receptors and cause mast cell degranulation Nonimmune-mediated mast cell activation includes Direct nonallergic activation of mast cells by certain drugs Drug-induced cyclooxygenase inhibition that activates mast cells by poorly understood mechanisms Activation by physical or emotional stimuli; mechanism is poorly understood but possibly involves the release of neuropeptides that interact with mast cells Etiology of Urticaria Urticaria is classified as acute (< 6 weeks) or chronic (> 6 weeks); acute cases (70%) are more common than chronic (30%).