Erythema is an abnormal redness of the skin due to dilation of the superficial capillaries of the skin causing inflammation. It can result from many different causes, diseases of the skin and some systemic diseases.
Simple erythema can be caused by cold, heat (as in contact with a heating pad or hot water bottle), or solar radiation. In its more severe forms it generally presents in one of two ways: (1) Erythema multiforme is an allergic (hypersensitivity) inflammatory reaction, often associated with herpes virus infections; (2) Erythema nodosum is an allergic (delayed-hypersensitivity) reaction that results in the formation of tender, deep red bumps on the arms or legs, particularly on the shins. It is most often associated with infections, the use of oral contraceptives and inflammatory bowel disease (such as Crohn's disease). However, the actual cause is identified in only about 50% of cases. Risk factors include heredity, female sex, recent travel, or a recent history of bacterial or fungal infection.