Allergies Health Article

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