Hives and rashes are often difficult to tell apart because they share many of the same symptoms and causes. Hives, called urticaria after the Latin word for nettle, presents as red, often itchy, raised welts on the skin. Rashes, also called dermatitis, are patches of irritated skin that may include swelling, redness, itching and/or burning.
Allergies
Hives are usually caused by an allergic reaction to such things as food or medicine. When a person comes into contact with an allergen--a substance to which he is allergic--the body releases a chemical compound called histamine into the bloodstream as part of its immune response. Histamine is present in many parts of the body, including mast cells. Mast cells are present in large quantities in connective tissue, including in the skin. When it is released in response to contact with an allergen, histamine causes nearby tissues to swell. Further, it initiates the other typical symptoms of an allergic reaction, such as inflammation, mucus production, a narrowing of the air passages and congestion.
Fresh foods cause more hive outbreaks than do cooked foods. The most common foods that cause hives include nuts, wheat products, tomatoes and milk products. Certain food additives and/or preservatives can cause an outbreak in some individuals.
Irritants
Rashes are often caused by contact with an irritating substance. They can develop after long-term contact with a mildly irritating substance, such as a soap or detergent, or after short contact with an especially harsh irritant, such as an acid or alkali. The rash is usually limited to the skin area that was in contact with the substance. Costume jewelry, for instance, often contains nickel, a metal that causes an allergic response in about 10 percent of women, according to the Cleveland Clinic. Those affected break out in a rash around the neckline or on the ears when exposed to jewelry made of nickel in those areas.
Infection
Chickenpox causes skin to break out in a red, itchy rash. Caused by the highly contagious herpes zoster virus, the chickenpox rash usually begins to form on the stomach and then spreads to the rest of the body.
Ringworm, jock itch and athlete's foot are rashes caused by a fungal infection. The fungus thrives in dark, warm, moist areas, such as in creases of the skin, in the groin area and between the toes, particularly the two smallest toes.
Environmental Factors
Physical urticaria is a hive caused by exposure to certain environmental factors and by direct physical stimulation of the skin. Some people break out into hives when skin is exposed to sunlight, wind, heat and/or cold. Others do so in response to exercise, vibration of the skin or pressure on the skin. This type of hive is usually limited to the area where exposure occurred. Dermatographism is a hive variant that occurs after skin has been stroked or scratched.
