Allergy symptoms vary depending on what is causing the reaction and the part of the body where the reaction occurs. Symptoms can include:

• runny nose
• tearing eyes, burning or itching eyes
• red eyes, conjunctivitis
• swollen eyes
• itching nose, mouth, throat, skin, or any other area
• wheezing
• coughing
• difficulty breathing
• hives (skin wheals)
• skin rashes
• stomach cramps
• vomiting
• diarrhea
• headache

The history of your symptoms is important in diagnosing all allergies, including whether the symptoms vary according to time of day, season, exposure to pets and other potential allergens, and diet changes. Severe reactions often develop very quickly after exposure, such as eating nuts or getting stung.

Allergy testing may be required to determine if your symptoms are an actual allergy or caused by other problems. For example, eating contaminated food (food poisoning) may cause symptoms that resemble food allergies. Some medications (such as aspirin, ampicillin, and others) can produce non-allergic reactions, including rashes, that resemble drug allergies but are not true allergies.

Tests that may reveal the specific allergens include:

• Skin testing -- the most common method of allergy testing. This may include intradermal, scratch, patch, or other tests. Skin testing may even be an option for young children and infants, depending on the circumstances.
• Blood test -- also called RAST (radioallergosorbent), this measures the levels of allergy antibody, IgE, produced when your blood is mixed with a series of allergens in a laboratory. If you are allergic to a substance, the IgE levels may increase in the blood sample. The blood test may be used if you have existing skin problems like eczema, if you're on medications that are long-acting or you cannot stop taking, if you have a history of anaphylaxis, or if you prefer not to have a skin test.
• "Use" or "elimination" tests -- suspected items are eliminated and/or introduced while the person is observed for response to the substance. This is often used to check for food or medication allergies.
• Eyelid -- Occasionally, the suspected allergen is dissolved and dropped onto the lining of the lower eyelid (conjunctiva) as a means of testing for allergies. (This test should only be done by a physician, never the patient, since it can be harmful if done improperly.)
• Reaction to physical stimuli -- application of heat, cold, or other stimulation, and then look for an allergic response.

Other tests that may reveal allergies include:

• Antibody/immunoglobulin (particularly IgE) levels -- when these are elevated, it indicates a "primed" immune system.
• CBC -- may reveal an increase in eosinophils.
• Complement levels -- may be abnormal.

Christodoulopoulos P, Tulic MK, Kontolemos M, Hamid Q. Immunopathology of Allergic Airway Inflammation.In: Middleton's Allergy: Principles and Practice.  6th ed.Mosby, Inc., 2003; 501-511.

American Academy of Allergy, Asthma and Immunology. The Allergy Report page. Available at: http://www.theallergyreport.org/. Accessed March 23, 2005.