Overview of allergic reactions

[Hina Muqadas]

Allergic reactions (hypersensitivity reactions) are inappropriate responses of the immune system to a normally harmless substance.

Usually, allergies make the eyes water and itch, the nose run, the skin itch, rashes develop, and people sneeze.
Some allergic reactions, called anaphylactic reactions, are life threatening.
Symptoms suggest the diagnosis, and skin tests can help identify the substance that triggers the allergy.
People who have had severe allergic reactions should always carry a self-injecting syringe of epinephrine and antihistamine pills.
Avoiding the trigger is best, but if it is impossible, allergy shots, when given long before the exposure occurs, can sometimes desensitize the person.
Severe reactions require emergency treatment in the hospital.

Normally, the immune system—which includes antibodies, white blood cells, mast cells, complement proteins, and other substances—defends the body against foreign substances (called antigens). However, in susceptible people, the immune system can overreact when exposed to certain substances (allergens) in the environment, foods, or drugs, which are harmless in most people. The result is an allergic reaction. Some people are allergic to only one substance. Others are allergic to many. About one third of the people in the United States have an allergy.
Allergens may cause an allergic reaction when they land on the skin or in the eye or are inhaled, eaten, or injected. An allergic reaction can occur in several ways:

As part of a seasonal allergy (such as hay fever), caused by exposure to such substances as tree, grass, or ragweed pollens
Triggered by taking a drug (drug allergy)
Triggered by eating certain foods (food allergy)
Triggered by breathing in dust, animal dander, or molds (year-round allergy)
Triggered by touching certain substances (such as latex)
Triggered by insect bites or stings (as occurs in anaphylactic reactions and angioedema)

In many allergic reactions, the immune system, when first exposed to an allergen, produces a type of antibody called immunoglobulin E (IgE). IgE binds to a type of white blood cell called basophils in the bloodstream and to a similar type of cell called mast cells in the tissues. The first exposure may make people sensitive to the allergen (called sensitization) but does not cause symptoms. When sensitized people subsequently encounter the allergen, the basophils and mast cells with IgE on their surface release substances (such as histamine, prostaglandins, and leukotrienes) that cause swelling or inflammation in the surrounding tissues. Such substances begin a cascade of reactions that continue to irritate and harm tissues. These reactions range from mild to severe.