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.

Causes

Mast cells involved in allergic reactions capture and display an antibody, called immunoglobulin E (IgE), that binds to allergens. After the allergen is bound, mast cell granules release a variety of potent chemicals, including histamine, that are responsible for some of allergic symptoms.

Immunologists distinguish allergic reactions into two main types: immediate hypersensitivity reactions, which are mainly mast cell-mediated and occur within minutes of contact with allergen, and delayed hypersensitivity reactions, mediated by T cells (a type of white blood cells) and occurring hours to days after exposure.

Inhaled or ingested allergens usually cause immediate hypersensitivity reactions. Allergens bind to IgE antibodies on the surface of mast cells, which release the contents of their granules onto neighboring cells, including blood vessels and nerve cells. Histamine binds to the surfaces of these other cells through special proteins called histamine receptors. Interaction of histamine with receptors on blood vessels causes increased leakage, thereby producing fluid collection, swelling, and redness. Histamine also stimulates pain receptors, making tissue more sensitive and irritable. Symptoms last from one to several hours following contact.

In the upper airways and eyes immediate hypersensitivity reactions cause the runny nose and itchy, bloodshot eyes typical of allergic rhinitis. In the gastrointestinal tract these reactions lead to swelling and irritation of the intestinal lining, causing the cramping and diarrhea typical of food allergy. Allergens that enter the circulatory system may cause hives, angioedema, anaphylaxis, or atopic dermatitis.

Allergens on the skin usually cause delayed hypersensitivity reaction. Roving T cells contact the allergen, setting in motion a more prolonged immune response. This type of allergic response may develop over several days following contact with the allergen, and symptoms may persist for a week or more.

THE ROLE OF INHERITANCE. While allergy to specific allergens is not inherited, the likelihood of developing some type of allergy seems to be, at least for many people. If neither parent has allergies, then the chances of a child developing allergy is approximately 10–20%; when one parent has allergies, it is 30–50%; and when both have allergies, it is 40–75%. Allergy patients share a genetic predisposition to produce higher levels of IgE in response to allergens. Those who produce more IgE will develop a stronger allergic sensitivity.

COMMON ALLERGENS. The most common airborne allergens are the following:

• plant pollens
• animal fur and dander
• body parts from house mites (microscopic creatures found in all houses)
• house dust
• mold spores
• cigarette smoke
• solvents
• cleaners

Common food allergens include the following:

• nuts, especially peanuts, walnuts, and Brazil nuts
• fish, mollusks, and shellfish
• eggs
• wheat
• milk
• food additives and preservatives

The following types of drugs commonly cause allergic reactions:

• penicillin or other antibiotics
• flu vaccines
• tetanus toxoid vaccine
• gamma globulin

Common causes of contact dermatitis include:

• poison ivy, oak, and sumac
• nickel or nickel alloys
• latex

Insects and other arthropods whose bites or stings typically cause allergy include:

• bees, wasps, and hornets
• mosquitoes
• fleas
• scabies