Allergy symptoms vary, but may include:

• Breathing problems
• Burning, tearing, or itchy eyes
• Conjunctivitis (red, swollen eyes)
• Coughing
• Diarrhea
• Headache
• Hives
• Itching of the nose, mouth, throat, skin, or any other area
• Runny nose
• Skin rashes
• Stomach cramps
• Vomiting
• Wheezing

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
• Allergens that touch the eyes may cause itchy, watery, red, swollen eyes.
• Eating something you are allergic to can cause nausea, vomiting, abdominal pain, cramping, diarrhea, or a severe, life-threatening reaction
• Allergens that touch the skin can cause a skin rash, hives, itching, blisters, or even skin peeling
• Drug allergies usually involve the whole body and can lead to a variety of symptoms

The health care provider will perform a physical exam and ask questions such as when the allergy occurs.

Allergy testing may be needed to determine if the symptoms are an actual allergy or caused by other problems. For example, eating contaminated food (food poisoning) may cause symptoms similar to food allergies. Some medications (such as aspirin and ampicillin) can produce non-allergic reactions, including rashes. A runny nose or cough may actually be due to an infection.

Skin testing is the most common method of allergy testing. One type of skin testing is the prick test. It involves placing a small amount of the suspected allergy-causing substances on the skin, and then slightly pricking the area so the substance moves under the skin. The skin is closely watched for signs of a reaction, which include swelling and redness. Skin testing may be an option for some young children and infants.

Other types of skin tests include patch testing and intradermal testing. For detailed information, see:Allergy testing

Blood tests can measure the levels of specific allergy-related substances, especially one called immunoglobulin E (IgE).

A complete blood count (CBC), specifically the eosinophil white blood cell count, may also help reveal allergies.

In some cases, the doctor may tell you to avoid certain items to see if you get better, or to use suspected items to see if you feel worse. This is called "use or elimination testing." This is often used to check for food or medication allergies.

The doctor may also check your reaction to physical triggers by apply heat, cold, or other stimulation to your body and watching for an allergic response.

Sometimes, a suspected allergen is dissolved and dropped into the lower eyelid to check for an allergic reaction. This should only be done by a health care provider.