An allergy, or hypersensitivity reaction, occurs when the body's immune system overreacts to a substance that is normally harmless (allergen), such as mold, pollen, animal dander or dust mites. The white blood cells of an allergic individual produce an antibody called immunoglobulin E (IgE), which attaches to the allergen. This triggers the release of histamine and other inflammatory chemicals that cause allergic symptoms, such as runny nose, watery eyes and hives.
If the allergen is airborne, the allergic reaction will primarily affect the eyes, nose and lungs. If the allergen is ingested, the allergic reaction will primarily affect the mouth, stomach and intestines. If enough inflammatory chemicals are released, a reaction such as hives or rash could occur throughout the body. The most severe allergic reaction, known as anaphylaxis, can lead to low blood pressure, breathing difficulties, shock, and loss of consciousness, all of which can be fatal.
Allergies are extremely common, affecting more than 20% of Americans. The most common allergy triggers include pollen, dust mites, molds, animal dander, latex, foods and insect venom.
Allergy treatment depends on the type of allergy and severity of symptoms. Commonly used allergy medications include antihistamines, nasal sprays, decongestants, leukotriene inhibitors and allergen immunotherapy (allergy shots).

Related Terms

Allergen, allergic, allergic reaction, allergic response, allergy, allergy shots, anaphylactic reaction, anaphylaxis, animal dander, antibodies, antibody, antihistamines, dander, decongestants, dust, dust mites, histamine, hives, hypersensitivity, immune, immune defense system, Ig, IgE, immune-mediated, immune response, immune system, immunoglobulin, immunoglobulin E, immunotherapy, inflammation, latex, molds, leukotriene inhibitors, pet dander, pollen, radioallergosorbent test, RAST, sensitized, sensitization, skin test, trigger, white blood cells.

types of allergic reactions

Allergic reactions can be classified into four immunopathologic categories using various classification systems. The Gell and Coombs allergic classification system is based on the immune system's response to the allergen, not on the severity of the reaction.
Type I: Type I allergic reactions involve immunoglobulin E (IgE), which is specific for a particular drug, antigen or other allergen that triggers the allergic reaction. The allergen binds to the immunoglobulin on specific immune cells called basophils and mast cells. This binding results in the release of chemicals that cause inflammation in the body (such as histamine, serotonin, proteases, bradykinin generating factor, chemotactic factors from immune cells, leukotrienes, prostaglandins and thromboxanes) within 30 minutes of exposure. These chemical mediators cause allergy symptoms, such as urticaria (hives), runny nose, watery eyes, sneezing, wheezing and itching. This type of allergic reaction is often seen with penicillin, latex, blood products and vaccines.
Type II: This classification is called a cytotoxic reaction because it involves the destruction of the host cells. An antigen associated with a specific cell initiates cytolysis (breakdown of the cell) by an antigen-specific antibody, such as immunoglobulin G (IgG) or immunoglobulin M (IgM). This reaction often involves blood elements, such as red blood cells, white blood cells or platelets. It often occurs within five to 12 hours of exposure to the allergen, which may include penicillin, quinidine, phenylbutazone, thiouracils, sulfonamides or methyldopa.
Type III: This category involves the formation of an antigen-antibody immune complex, which deposits on blood vessel walls and activates cell components called complements. This causes a serum sickness-like syndrome, involving fever, swelling, skin rash and enlarged lymph nodes, in about three to eight hours. It may be caused by a variety of allergens, including penicillins, sulfonamides, intravenous (IV) contrast media and hydantoins.
Type IV: This classification involves delayed cell-mediated reactions. Antigens on the allergen release inflammatory mediators within 24 to 48 hours of exposure. This type of reaction is seen with graft rejection, latex, contact dermatitis and tuberculin reaction.

common triggers

Pollen: Each spring, summer and fall, plants release tiny particles called pollen into the air in order to reproduce. Pollen from plants, such as ragweed, can trigger allergy symptoms.
Dust: Dust mites are microscopic organisms that live in dust and in the fibers of household objects like pillows, mattresses and carpets. Dust mites prefer warm, humid areas. Household dust is a combination of potentially allergenic materials, including fibers from different fabrics, animal dander, bacteria, mold, fungus spores, food particles, bits of plants or other allergens.
Molds: Molds are parasitic, microscopic fungi (like Penicillium) that have spores that float in the air like pollen. Mold is a common trigger for allergies, and it is usually found in damp areas, such as the basement or bathroom, as well as outside in grass, leaf piles, hay or mulch. In some people, symptoms of mold allergy may be triggered or worsened after eating certain foods, such as cheese processed with fungi. Mold spores peak during hot, humid weather.
Animal dander: Animals secrete oily fluids from their skin, which contain allergens. These fluids collect on fur, feathers and other surfaces inside the home. Proteins in the animal's saliva also cause allergic reactions. The allergens are capable of triggering reactions for several months. Allergies to animals can take two or more years to develop, and symptoms may not subside until months after discontinuing contact with the animal.
Latex: Latex, a substance found in products like rubber gloves or condoms, can also trigger allergic reactions in sensitive people. A component of the latex substance itself is an allergen. In addition, the latex glove powder residue is an airborne allergen that causes upper airway allergic reactions in some people. Latex reactions may cause a potentially life-threatening allergic reaction called anaphylaxis. According to the American Academy of Allergy Asthma & Immunology (AAAAI) about 220 cases of anaphylaxis and three deaths per year are attributed to latex allergies.
Foods: Food allergens are those parts of foods, usually proteins, which lead to allergic reactions. Most allergens can still cause allergic reactions even after they are cooked or have been digested. However, some allergens (usually from fruit and vegetables) only cause allergic reaction if eaten raw. These reactions are usually limited to the mouth and throat. According to the AAAAI, six foods, including milk, peanuts, soy, eggs, wheat and tree nuts (like pecans and walnuts), cause 90% of food allergies in children. Children usually outgrow allergies to milk, eggs and soy, but peanut, tree nut, fish and shellfish allergies continue throughout adulthood.
Insect venom: The honeybee, yellowjacket, paper wasp, white-faced hornet (bald-faced hornet) and fire ant are among the most common insects that trigger insect sting or bite allergies. Since the stinger is a modified egg-laying apparatus, only females can sting.