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

Allergies are relatively common. Both environmental and genetics factors play a role.The immune system protects the body against harmful substances, such as viruses and bacteria. It also reacts to foreign substances known as allergens, which are generally harmless and in most people do not create a problem.

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

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

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

Allergy is not usually passed down through families. However, if both your parents have allergies, you are likely to have allergies. The chance is greater if your mother has allergies.

Allergies may make certain medical conditions such as eczema, asthma and sinus problems worse.