Hypersensitivity reactions in urticaria include type I, type II, and type III hypersensitivity reactions. Type I hypersensitivity reactions are caused by specific IgE binding to mast cells and basophil surface receptors, causing two types of cell degranulation to release histamine and other mediators, resulting in increased vascular permeability, capillary dilation, and smooth muscle contraction producing symptoms in the skin mucosa as well as the digestive and respiratory tracts. Type II hypersensitivity reactions are mostly seen in patients with IgA deficiency and can occur in transfusion reactions causing anaphylaxis. Type III allergic reactions are mainly urticarial vasculitis, mostly due to more antigens, immune complexes of antigens and antibodies deposited in the vessel wall, activation of complement to make mast cells as well as neutrophils release histamine and other inflammatory mediators, causing a disease of increased permeability and edema of the vessel wall.