Definition of Allergies:

An allergy is an exaggerated immune response or reaction to substances that are generally not harmful.

See also:

• Allergic conjunctivitis
• Allergic reactions
• Allergy testing
• Allergy to mold, dander, dust
• Atopic dermatitis (eczema)
• Contact dermatitis
• Drug allergies
• Food allergies
• Hay fever (allergic rhinitis)

Causes, incidence, and risk factors:

Allergies are relatively common. Both genetics and environmental factors play a role.

Allergy is caused by an oversensitive immune system, which leads to a misdirected immune response. The immune system normally protects the body against harmful substances, such as bacteria and viruses. It reacts to substances (allergens) that are generally harmless and in most people do not cause a problem.

But in a person with allergies, the immune response is oversensitive. When it recognizes an allergen, it releases chemicals, such as histamines. This causes itching, swelling, mucus production, muscle spasms, hives, rashes, and other symptoms, which vary from person to person.

What part of the body is contacted by the allergen plays a role in the symptoms you develop. For example, allergens that are breathed in often cause a stuffy nose, itchy nose and throat, mucus production, cough, or wheezing. A food allergen can cause nausea, vomiting, abdominal pain, cramping, diarrhea, or a severe, life-threatening reaction. Allergies to plants often cause a skin rash. Drug allergies usually involve the whole body and can lead to a variety of symptoms.

Certain medical conditions such as sinus problems, eczema, and asthma are linked to allergies.

Common allergens include pollen, mold, pet dander, and dust. Food and drug allergies are common. Allergic reactions can also be caused by insect bites, jewelry, cosmetics, and other substances.

Some people have allergy-like reactions to hot or cold temperatures, sunlight, or other environmental triggers. Sometimes, friction (rubbing or roughly stroking the skin) will cause symptoms.

Whether or not you are likely to develop allergies is often passed down through families. If both your parents have allergies, you are likely to have allergies. The chance is greater if your mother has allergies.