Diagnosing an allergy isn't always easy. A number of conditions can look like an allergy, but don't involve the immune system. For example, vasomotor rhinitis causes nasal congestion and a runny nose, just like an allergy to dust mites or ragweed pollen, but people with this condition are sensitive to irritants or temperature changes within their nose, and test negative to inhaled allergens. Their symptoms can develop or worsen in response to cigarette smoke, perfumes, paint or gasoline fumes, or to changes in the weather. Vasomotor rhinitis is believed responsible for between 60 percent and 70 percent of all cases of year-round respiratory symptoms medically known as chronic rhinitis. Another problem that might explain a persistent runny nose is excessive use of nasal decongestant sprays.

When you first consult a doctor, be prepared to answer a few questions about your lifestyle and medical history. Your doctor may ask about your diet and your surroundings at home and at work. Some questions you might anticipate include:

• Do you or members of your family smoke? Do coworkers smoke? What other aspects of your home or work environment might contribute to your symptoms — for example, fumes from paint, gasoline or other chemicals?

• Are your symptoms seasonal or year-round? Do they worsen during certain months? Are you ever entirely free of them?

• Do your symptoms occur when visiting a home with pets? If you have pets at home, do your symptoms improve if you spend a week or longer away from home? Do your symptoms return or worsen when you get home?

• Do your eyes or skin itch and redden after handling a pet?

• Do you sneeze, cough, wheeze, or develop itching or dripping in your nose or eyes when the carpets are being vacuumed?

• Do you develop symptoms when you enter a damp basement or home that is not well ventilated?

• Are your symptoms better or worse when you are at work? Do your symptoms get better when you take time off work or are on vacation?

• Do either or both of your parents have allergies or asthma? Did they start at an early age?

• Do your allergies appear worse during times of stress?

The answers to these questions, as well as the details of your medical history, will help your doctor to judge whether an allergy is responsible for your symptoms, and to advise you about what type of allergy you might have. For example, if your symptoms occur only during the late summer and early fall, there's good reason to suspect that you are allergic to ragweed pollen. If so, the doctor probably will recommend steps you should take to avoid exposure to pollen and recommend or prescribe medications to help control your symptoms.

Your primary care doctor should be able to treat most people with seasonal allergy (hay fever). But if you need tests to identify what you're allergic to or to plan whether allergy shots might be helpful, you may need to see an allergy specialist. If your doctor thinks you would benefit from allergy shots ( immunotherapy ), you may be referred to a specialist for treatment.