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. Hypersensitivity (allergic) and inflammatory skin disorders are caused by immune system reactions that involve the skin. These disorders include the following:
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 sensations. Sometimes an immune reaction is triggered by substances a person touches or eats or by a drug a person takes, but many times doctors do not know why the immune system reacts to produce a rash.
Some rashes occur mostly 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, so they 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 the skin affected by the rash is removed with a blade for examination under a microscope. Also, if the doctor suspects contact dermatitis is the cause, patch testing may be done.