Allergy:A misguided reaction to foreign substances by theimmune system, the body system of defense against foreign invaders, particularly pathogens (the agents ofinfection). Theallergic reactionis misguidedin这些外来物质通常是无害的．The substances thattriggerallergyare calledallergen．Examples include pollens,dust mite, molds,danders, and certain foods. Peopleproneto allergies are said to be allergic oratopic．
Althoughallergiescan develop at any age, the risk of developingallergiesisgenetic．It is related to onesfamily historyofallergy．If neither parent is allergic, the chance for allergies is about 15%. If one parent is allergic, the risk increases to 30% and if both are allergic, the risk is greater than 60%.
Allergens cause the production ofimmunoglobulin E(IgE), anantibodythat all of us have in small amounts. Allergic persons, however, produce IgE in abnormally quantities. Normally, this antibody is important in protecting us from parasites, but not from other allergens. During the sensitization period in allergy, IgE is overproduced. It coats certain potentially explosive cells that contain chemicals includinghistamine．These chemicals, in turn, causeinflammationand the typical allergic symptoms. This is how theimmunesystem becomes misguided and primed to cause an allergic reaction when stimulated by anallergen．
The most common allergic conditions include hay fever (allergic rhinitis), asthma, allergic eyes (allergic conjunctivitis), allergic eczema, hives (urticaria), and allergic shock (also called anaphylaxis andanaphylactic shock). For a thumbnail sketch of each of these conditions:
Hayfever(allergic rhinitis)is the most common of the allergic diseases and refers to seasonalnasalsymptoms that are due to pollens. Yearroundor perennialallergic rhinitisis usually due to indoor allergens, such as dust mites or molds. Symptoms result from the inflammation of the tissues that line the inside of thenose(mucuslining or membranes) after allergens are inhaled.Adjacentareas, such as the ears, sinuses, andthroatcan also be involved. The most common symptoms include:
- Runny nose
- Stuffy nose
- Sneezing
- Nasal itching (rubbing)
- Itchy ears and throat
- Post nasal drip (throat clearing)
Asthmais a breathing problem that results from the inflammation andspasmof the lung's air passages (bronchialtubes). The inflammation causes a narrowing of the air passages, which limits the flow of air into and out of thelungs．Asthmais most often, but not always, related to allergies. Common symptoms include:
- Shortness of breath
- Wheezing
- Coughing
- Chesttightness
Allergic eyes (allergic conjunctivitis)is inflammation of thetissuelayers (membranes) that cover the surface of the eyeball and the undersurface of theeyelid．The inflammation occurs a result of an allergic reaction and features:
- Redness under the lids and of theeyeoverall
- Watery, itchy eyes
- Swelling of the membranes
Allergiceczemais an allergic rash that is usually not caused byskincontact with an allergen and features the following symptoms:
- Itching, redness, and or dryness of the skin
- Rash on the face, especially children
- Rash around the eyes, in the肘creases, and behind the knees, especially in adults
Hives (urticaria)are skin reactions that appear as itchy swellings and can occur on any part of the body. Hives can be caused by an allergic reaction, such as to afoodormedication, but they also may occur in non-allergic people. Typical hive symptoms are:
- Raised red welts
- Intense itching
Allergic shock(过敏反应或anaphylactic shock) is a life-threatening reaction that canaffecta number of organs at the same time. It typically occurs when the allergen is eaten (for example, foods) or injected (for example, a bee sting). Allergic shock is caused by dilated and "leaky"bloodvessels, which result in a drop inblood pressure．followi的部分或全部ng symptoms may occur:
- Hives or reddish discoloration of the skin
- Nasalcongestion
- Swelling of the throat
- Stomach pain, nausea, vomiting
- Shortness of breath, wheezing
- Low blood pressure or shock
