Allergies Health Article

Definition

Allergies are abnormal reactions of the immune system that occur in response to otherwise harmless substances.

Description

Allergies are among the most common of medical disorders. About one quarter of all Americans suffer from some form of allergy, such as asthma, allergic rhinitis (hay fever), or atopic dermatitis (eczema). Allergy is the single largest reason for school absence; allergic rhinitis alone is responsible for two million school day absences annually. Allergies cause one out of nine physician visits and are responsible for significant losses of productivity in the workplace.

An allergy is a type of immune reaction. Normally, the immune system responds to bacteria, viruses, or particles—such as pollen or dust—by producing antibodies (specific proteins) capable of binding to antigens (identifying molecules) on the foreign particle. The interaction between the antibody and antigen sets off a series of reactions designed to protect the body from infection. When this same series of reactions is triggered by harmless, everyday substances, it is known as an allergy, and the offending substance is called an allergen.

Allergens enter the body through four main routes: the airways, skin, gastrointestinal tract, and the circulatory system.

• Airborne allergens cause the sneezing, runny nose of allergic rhinitis. Airborne allergens can also affect the lining of the lungs, causing asthma, or the conjunctiva of the eyes, causing conjunctivitis (pink eye).
• Allergens in food may cause itching and swelling of the lips and throat, cramps, and diarrhea. When absorbed into the bloodstream, they may cause urticaria (hives) or more severe reactions such as swelling of the skin, mucous membranes, organs, and brain (angioedema). Some food allergens may cause anaphylaxis, a potentially life-threatening condition marked by tissue swelling, airway constriction, and a sudden drop in blood pressure.
• Allergins that come in direct contact with the skin can cause reddening, itching, and blistering (contact dermatitis). Skin reactions can also occur from allergens introduced through the airways or gastrointestinal tract. This reaction is known as atopic dermatitis or eczema.
• Injection of allergens, from insect bites and stings or drug administration, can introduce allergens directly into the circulatory system. There they may cause systemwide responses (including anaphylaxis), as well as local reactions of swelling and irritation at the injection site.

Individuals with allergies are not equally sensitive to all allergens. For example, some may have severe allergic rhinitis but no food allergies, others are extremely sensitive to nuts but not to any other food. Allergies may worsen over time. For example, childhood ragweed allergy may progress to year-round dust and pollen allergy. On the other hand, an individual may lose allergic sensitivity. Infant or childhood atopic dermatitis almost always disappears with advancing age. More commonly, an individual's apparently diminished sensitivity may instead be attributable to reduced exposure to allergens or an increased tolerance for allergy symptoms.