Physical Allergy
A physical allergy is an allergic reaction triggered by a physical stimulus.
A physical allergy differs from other allergic reactions because the trigger is a physical stimulus. The physical stimulus can be cold, sunlight, heat, other stimuli that cause sweating (such as emotional stress or exercise), vibration, a minor injury (such as that due to scratching), or physical pressure. For some people, symptoms occur only in response to a physical stimulus. For some people who have other allergies, a physical stimulus makes symptoms worse.
What causes this type of allergic reaction is not understood. One theory suggests that the physical stimulus changes a protein in the skin. The immune system mistakes this protein for a foreign substance and attacks it. Sensitivity to sunlight (photosensitivity) is sometimes triggered by the use of drugs, such as antibiotics, or other substances, including some cosmetics such as skin creams, lotions, and oils. A few people who are sensitive to cold have abnormal proteins (called cryoglobulins or cryofibrinogen) in the blood. Sometimes the presence of these proteins indicates a serious disorder such as cancer, a connective tissue disorder, or chronic infection.
Itching, skin blotches, hives, and angioedema are the most common symptoms. The symptoms tend to develop within minutes of exposure to the physical stimulus.
When people who are sensitive to heat are exposed to heat or engage in any activity that causes sweating, they may develop small, intensely itchy hives that are surrounded by a ring of redness--a condition called cholinergic urticaria.
When people who are sensitive to cold are exposed to cold, they may develop hives, asthma, a runny nose, nasal stuffiness, or swelling of tissues under the skin (angioedema). Rarely, a widespread anaphylactic reaction occurs.
Diagnosis and Treatment
The diagnosis is based on symptoms and the circumstances under which they occur. To diagnose reactions caused by cold, doctors place an ice cube on the skin for 4 minutes, remove the ice cube, then watch for the development of a hive.
The best treatment is to avoid the stimulus that causes the physical allergy. Not using cosmetics and skin creams, lotions, and oils for a while enables the person to determine whether one of these substances may be worsening the allergy.
An antihistamine can usually relieve itching. Cyproheptadine tends to work best for hives caused by cold, and hydroxyzine for hives caused by heat or emotional stress. People who are very sensitive to sunlight should use a sunscreen and avoid exposure to the sun as much as possible.
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