DEAR DOCTOR K:

A friend recently revealed that she is allergic to sunshine. I was so surprised -- what does this mean, exactly?

DEAR READER:

I agree that it's a strange-sounding concept, but sun allergy is a real phenomenon. A sun allergy is an immune system reaction to sunlight.

Sun allergy usually results in an itchy red rash after sun exposure. This is most common on the "V" of the neck, the back of the hands, and the outside surface of the arms and the lower legs.

Sun allergies are triggered by the effect of the sun on natural chemicals in the skin. The sunlight changes the shape of the chemicals, which makes them look "foreign" to the immune system. As a result, the body activates its immune defenses. The resulting allergic reaction can take the form of a rash, tiny blisters or, rarely, some other type of skin eruption.

Sun allergies occur only in certain sensitive people. They can be triggered by only a few brief moments of sun exposure. Scientists don't know why some people develop a sun allergy and others don't.

There is evidence, however, that some forms of sun allergy are inherited. For instance, people of American Indian background, including the native populations of North, South and Central America, have a higher rate of inherited sun allergy.

There are a variety of types of sun allergy. The most common is polymorphous light eruption (PMLE), which usually appears as an itchy rash on sun-exposed skin.

But some people have more severe sun-related symptoms. This can mean hives, blisters or small areas of bleeding under the skin. In cases like these, a doctor will need to assess symptoms, examine a person's medical history, family history, and perform a simple examination of the skin. Sometimes additional tests may be necessary.

People with a sun allergy must protect their skin from exposure to sunlight. Wearing sunscreen is a must for all exposed areas, including the lips. It should have an SPF of 15 or above and protect against both ultraviolet A and ultraviolet B rays.

Those with a sun allergy should limit time spent outdoors when the sun is at its peak and also wear long pants, a shirt with long sleeves and a hat with a wide brim.

Red-flag symptoms, such as developing hives along with swelling around the eyes or lips, faintness, or difficulty breathing or swallowing, can be signs of a life-threatening allergic reaction that requires immediate medical attention.

Finally, some rashes that occur with exposure to the sun are caused by medicines. For example, tetracyclines, sulfonamides (sulfa drug antibiotics), thiazide diuretics and fluoroquinolone antibiotics can cause rashes when skin is exposed to sun.

The way to deal with all of these problems is the same as how to prevent aging of the skin and skin cancer: Protect yourself with sunblock and clothes. This just becomes more important if you have a sun allergy, or are taking a medicine that makes you sun-sensitive.

(Dr. Komaroff is a physician and professor at Harvard Medical School. Go to his website to send questions and get additional information: www.AskDoctorK.com.)

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