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Dermatophytid Reaction

(Identity or Id Reaction)

By Denise M. Aaron, MD, Assistant Professor of Surgery;Staff Physician, Dartmouth-Hitchcock Medical Center;Veterans Administration Medical Center, White River Junction

A dermatophytid reaction is the body's reaction to a dermatophyte (fungal) infection and is a skin eruption that appears on an area of the body that is not the area where the infection first began.

A dermatophytid reaction is not actually a type of dermatophytosis. Rather, a fungal infection on one area of the body can cause an allergic skin eruption to appear on another area of the body that is not infected. For example, a fungal infection on the foot may cause an itchy, bumpy rash to appear on the fingers. These eruptions (dermatophytids, or identity or id reactions) are allergic reactions to the fungus. They do not result from touching the infected area. The eruptions may appear on many different areas of the body at once.

The eruptions are typically itchy. They may appear as

  • Small, fluid-filled spots (on the hands or feet)

  • Solid bumps

  • Red, raised patches

  • Deep, raised, bruiselike areas on the shins

  • Pinkish red spots that resemble targets

  • Red, raised swellings (hives)

Doctors base the diagnosis of dermatophytid reactions on an examination of skin scrapings. Scrapings taken from the areas that have the dermatophyte infection show the fungus, but scrapings taken from the areas that have the dermatophytid reaction do not. This combination of findings indicates that the second (separate) eruption is a dermatophytid reaction.

The dermatophytid reaction goes away once the dermatophyte infection has been cured. To relieve symptoms of dermatophytid reactions, doctors give corticosteroid creams, anti-itch drugs taken by mouth (such as hydroxyzine), or both.

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