Severe allergic reactions can manifest as rashes. The type of rash is dependent on the allergen to which you're exposed, the duration of exposure, and the mechanism of your body's response. Usually allergic rashes can be treated with antihistamines, but more severe reactions may require other forms of treatment. If you're not sure about the nature of your allergic rash, you should seek medical advice and treatment.
Hives
Hives, or urticaria, are raised, red, itchy welts on the skin. They are your body's response to allergens such as a certain food, a drug, or even an environmental agent like pollen. Hives are the result of Type 1 hypersensitivity and occur when your body releases massive amounts of histamine and other inflammatory mediators. These inflammatory mediators cause redness, swelling, itching and sometimes pain. Characteristic hives manifest blanching, or a central whitening of the welt when you apply pressure. In more severe cases of hives, the lesions can run together, creating large areas of itchy redness. Hives can usually be managed with the use of antihistamines.
Angioedema
Angioedema, like hives, are a result of a Type 1 hypersensitivity reaction. Unlike hives, which occur on the surface of the skin, angioedema occurs underneath the skin, usually around the mouth and eyes. Angioedema can be dangerous if the tongue or epiglottis swells up and blocks your airway. If it does, you should seek emergency medical care immediately; intubation may be necessary.
Erythema Multiforme
Erythema multiforme is a type of hypersensitivity to drugs or infections that is mediated by damage to the blood vessels and later, the skin and soft tissues. This is a serious condition that may present with fever, joint pains, general illness, and skin lesions that can range from blisters and vesicles to peeling and desquamation of the skin. Erythema multiforme is usually treated by antihistamines and corticosteroids.
Stevens-Johnson Syndrome
Stevens-Johnson syndrome, or erythema multiforme major, is a severe and sometimes life-threatening dermatological manifestation of a severe allergy. It is characterized by fever, extremely painful skin lesions, joint pains, bloodshot eyes, and unstable vital signs. Treatment, depending on the severity of the reaction, can range from antihistamines, analgesics and corticosteroids, to admission to an intensive care or burns unit, fluid resuscitation and intravenous immunoglobulins.
