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Introduction to Hypersensitivity and Inflammatory Skin Disorders

By Wingfield E. Rehmus, MD, MPH, Clinical Assistant Professor of Pediatrics, Associate Member of Department of Dermatology, University of British Columbia; , BC Children’s Hospital, Division of Dermatology

Hypersensitivity and inflammatory skin disorders are caused by immune system reactions that involve the skin.

The immune system plays a vital role in maintaining the health of all the tissues of the body. The immune system reacts to invaders, such as microorganisms, foreign substances, or cancer cells, and triggers inflammation to attack these invaders. Usually the immune system reaction protects the body and aids healing. However, sometimes an immune system reaction is misdirected at healthy tissues and causes intense inflammation and damage.

Skin can be involved in a variety of immune system reactions, many of which cause rashes. The word "rash" refers to changes in skin color (such as redness), and/or texture (such as bumps or swelling). Many rashes itch, such as those that often develop after an allergic (hypersensitivity) reaction, but some rashes are painful or cause no symptoms. Sometimes an immune reaction is triggered by substances a person touches or eats, but many times doctors do not know why the immune system reacts to produce a rash.

Some rashes occur mostly in children (see Rashes in Children), whereas others almost always occur in adults.

The diagnosis of most hypersensitivity rashes is based on the appearance of the rash. The cause of a rash often cannot be determined by blood tests, and tests of any kind are not usually done. However, persistent rashes, particularly those that do not respond to treatment, may lead the doctor to do a skin biopsy, in which a small piece of skin is surgically removed for examination under a microscope. Also, if the doctor suspects contact dermatitis is the cause, skin tests may be done.

The immune system plays a major role in many other skin disorders, including dermatitis, sunlight reactions (photosensitivity reactions), and blistering disorders, which are discussed elsewhere.