Diseases & Conditions
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. Physical stimuli include the following: Cold Sunlight Heat or other stimuli that cause sweating (such as emotional stress or exercise) Vibration Minor injuries (such as those due to scratching) 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 an example. Ultraviolet light changes proteins in the skin, which the body then identifies as foreign and attacks. Photosensitivity is sometimes triggered by the use of drugs (such as antibiotics), some cosmetics (such as skin creams, lotions, and oils), or other substances.
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 swelling of tissues under the skin (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 angioedema. Rarely, a widespread anaphylactic reaction occurs.
Diagnosis and Treatment
The diagnosis is based on symptoms and the circumstances in 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. People may be advised not to use cosmetics and skin creams, lotions, and oils for a while to help determine whether one of these substances may be worsening the allergy.
The best treatment is to avoid the stimulus that causes the physical allergy. For example, people who are very sensitive to sunlight should use a sunscreen and avoid exposure to the sun as much as possible.
An antihistamine can usually relieve itching. The most effective treatments are cyproheptadine Some Trade Names PERIACTIN for hives caused by cold and hydroxyzine Some Trade Names ATARAX VISTARIL for hives caused by heat or emotional stress.
Last full review/revision September 2008 by Peter J. Delves, PhD
Source: The Merck Manual Home Edition