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People with allergies develop characteristic symptoms such as itchy eyes and nose, sneezing, wheezing, hives, and other symptoms within a few minutes following exposure to foreign proteins called allergens. These allergens can be pollens, animal dander, food, medications, or insect venom.

Allergic reactions result because the immune systems of people with allergies form a particular type of antibody, the IgE antibody, to these allergens. Unlike other types of antibodies, IgE antibody has the ability to activate certain immune cells, called mast cells, to release histamine and other pro-inflammatory substances. Only genetically predisposed individuals (approximately 15% of the US population) make IgE antibody against foreign proteins.

The part of the body affected and the nature of the resulting symptoms depends on the type of allergen exposure, that is, whether the allergens were inhaled, made contact with skin, or were ingested. The severity of allergic reactions varies widely.

Allergic reactions can be mild, like minor itching, or very serious. The most serious allergic reaction is an anaphylactic reaction, where the reaction of the body to the allergen causes a rapid drop in blood pressure. When a person experiences an anaphylactic reaction, they need medical attention right away, or they could lose consciousness or die.