(See also Overview of Allergic Reactions.)
A physical allergy differs from other allergic reactions because the trigger is a physical stimulus. Physical stimuli include the following:
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 a physical 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.
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.
The diagnosis of physical allergies 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 for physical allergies 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 for hives caused by cold and hydroxyzine for hives caused by heat or emotional stress.