An allergy is defined as an overreaction to usually harmless substance that the immune system determines is harmful, even if it is not. When the immune system is challenged with an allergen (substance which causes an allergic reaction) it becomes sensitized, committing the allergen memory. From that moment forward, the immune system recognizes the allergen and sets up a defense by releasing the chemical into the blood that may cause various symptoms.
Diagnosis of allergy is mainly depends upon carful history, habits, symptoms, physical examination. In case of food allergy elimination diet and food diary should be maintained. Laboratory tests mainly include skin and blood tests.
A doctor will base the diagnosis on a physical examination and your symptoms. In most cases, a doctor will suspect that your problem is a drug allergy if you have a history of allergic reactions after the use of certain medications.
Skin tests sometimes can be used to determine if someone has an allergy to a medication. Penicillin skin testing, for example, involves an injection just beneath the skin of a small amount of one part of the penicillin molecule. People with a penicillin allergy will develop a reaction at the site of injection that can be measured. However, these tests are not always reliable since only a part of the penicillin molecule is injected. This test often is used for a person who needs a penicillin-like drug to treat a serious infection.