A condition in which the immune system reacts abnormally to a foreign substance.
Allergies stem from an inappropriate reaction of the immune system to certain proteins. These proteins are known as allergens, and they are usually common and harmless substances such as pollen, mold spores, animal dander, dust, various foods, insect venoms, or medicines.The immune system is designed to identify intruders within the body and get rid of them. For an allergic person, however, the immune system has a hard time identifying which are the dangerous intruders, and which are okay. An allergic person has a hypersensitive immune system, which singles out a harmless substance (like the allergens mentioned above), then attacks it.
The type of symptoms an allergy sufferer shows depend on the type of allergen. Respiratory allergens cause symptoms like sneezing, sniffling, wheezing, coughing, runny nose, itchy eyes, sore throats, etc. Skin irritants can cause eczema, hives, or other reactions. Responses to food allergens vary greatly, and in severe cases can cause a possibly fatal condition, called anaphylaxis.