Eczema is a group of skin conditions characterized by inflammation and irritation. Itching is the most common symptom in all types of eczema. The skin may appear dry, scaly or red. In darker-skinned people, the affected skin may change color. While any part of the body can become affected, the face, wrists, elbows and knees are most commonly involved. Between 10 and 20 percent of infants in the United States suffer from eczema, but most outgrow the condition by their second birthday. In the remainder, eczema can be a chronic and even lifelong condition.

Allergic reactions are thought to play a central role in the itching and skin changes that occur in eczema. Predisposing factors for eczema are a family history of eczema, allergy to certain foods or airborne allergens, contact with certain substances, and emotional stress.

The most common type of eczema is atopic dermatitis, a chronic condition in which the skin is extremely itchy and appears dry or scaly. Cracks and blisters may appear in more severe cases, and the skin may become more vulnerable to infection from various germs. Atopic eczema is often episodic, with exacerbations interspersed with periods during which the skin appears completely normal. Atopic dermatitis is sometimes seen in people who have other allergic conditions, such as asthma.

There are several other types of eczema. Contact eczema is characterized by a localized skin reaction to either an allergen, such as poison ivy, or to an irritant compound such as detergent. A large number of allergens and compounds have been implicated in contact eczema. In seborrheic eczema, oily, scaly patches form on the skin of the scalp, causing dandruff. The face, shoulders and back may also be involved. Dyshidrotic eczema affects the skin of the palms and soles, with blistering and severe itching and burning. In nummular eczema, coin-shaped patches appear on the back, buttocks, legs or arms.