Immunoglobulin E (IgE) is firmly bound to mast cells and only trace amounts are normally present in serum. When antigen (allergen) crosslinks two IgE molecules, the mast cell is stimulated to release histamine and other vasoactive amines. These vasoactive amines are responsible for the vascular permeability and smooth muscle contraction occurring in such allergic reactions as hay fever, asthma, urticaria, and eczema.