IgE antibodies are the immunoglobulins involved in allergic reactions. Producing excessive amounts of IgE antibodies is one characteristic of individuals with atopic hypersensitivity. The term 'atopy' can be simply defined as a tendency to develop classic allergic diseases which includes hay fever (rhinitis and conjunctivitis), asthma, eczema, and urticaria. Each allergen (allergic substance) stimulates the production of a specific IgE. For example, in some individuals, bluegrass pollen (an allergen) will trigger the formation of a specific IgE that reacts only with this pollen type. The interaction of allergens with the cell-bound allergen specific IgE stimulates these cells to release histamine, thereby initiating the allergic reaction. Total serum IgE can be measured by nephelometry, while specific IgE levels are detected by more sensitive techniques such as radioallergosorbent testing (RAST). In normal individuals, the concentration of IgE antibodies in their serum is found to be less than 0.001% of the total antibodies level. Patients with atopic hypersensitivity have elevated levels of IgE and it is noticed that the more intense the allergy exposure, the higher the total IgE level is. Therefore, the IgE level is related to the degree of immune stimulation, as well as to the number of allergens to which the patient is allergic to.