According to Wikipidia, allergies are a disorder of the immune system. Allergies are basically anything that your body rejects. Those items that your body rejects are called allergens.

To find out what allergens your child’s body is rejecting the first step would be taking him/her to an allergist. Usually a blood test is ordered to check IgE (immunoglobulin E) levels in the system. The immunoglobulin E is a type of antibody which determines how allergic or hypersensitive your child is. High levels of IgE are found in most allergic kids. That simply means that the antibodies are fighting or rejecting an allergen. These allergens can be anything, from environmental to food. To determine the most common allergens a skin test is conducted. With the results in hand it is easier to prevent, treat and cure allergies.