What is an allergy?
The term “allergy” is used many ways, sometimes loosely, as with many people who say they are allergic to something if it makes them feel unwell in any way. The clinical definition of an allergy, however, is hypersensitivity (abnormal sensitivity) to a foreign substance (allergen) that is normally harmless. An allergic reaction is the body’s way of trying to get rid of an allergen. Some of the more common allergies include hay fever; allergic asthma; eczema in infants; drug allergies; and food allergies. Hereditary factors are thought to play a role in the development of allergies. An allergy is different from an intolerance. For example, a food intolerance may be caused by a lack of a specific substance in the body, whereas an allergy is caused by an immune response by the body.
What are the symptoms of allergic reactions?
People respond to allergies in different ways and the symptoms vary depending on the type of allergy. Hay fever is a common allergy that causes a variety of symptoms, including as a runny or stuffy nose, sneezing, and eye symptoms such as watering, redness, itchiness or a gritty feeling. Allergies can also cause asthma-type wheezing in the chest. Most asthma-type reactions cause narrowing of the airways, which makes breathing more difficult.
Another fairly common allergen is metal, such as nickel. Nickel is often found in jewelry and watch straps and can cause an eczema-like rash (eg, red, itchy, raised blisters) usually referred to as contact dermatitis. True food allergies, as opposed to food intolerances, are relatively rare. Reactions range from mild (eg, gastric upset and diarrhea) to life-threatening (swelling of the lips, tongue and throat, and severe asthma). Celiac disease (or gluten intolerance) is a particular condition caused by an allergy to gluten, a constituent of wheat. Some food additives such as tartrazine can cause hives or asthma.
An allergic substance may cause a serious, immediate reaction known as anaphylaxis. This can happen in response a drug, bee or wasp stings, or in rare cases to certain foods such as nuts or shellfish. Symptoms of anaphylaxis occur very suddenly and can include the following: generalized swelling; swelling of the throat causing difficulty in breathing; asthma symptoms; itchy rash; and faintness and unconsciousness due to low blood pressure. Anaphylaxis can be life-threatening and requires immediate emergency treatment.