Rashes, also known as dermatitis, are the result of inflammation of the skin. This inflammation leads to redness and itching. Rashes generally aren't considered a medical emergency, but they can cause serious discomfort. Rashes can also signal that a patient is allergic to a substance, which can be important to avoid serious complications of allergies, such as anaphylaxis.
Contact Dermatitis
One cause of a skin rash, according to the Mayo Clinic, is contact dermatitis. Contact dermatitis describes a rash that appears in response to a substance touching the skin that causes irritation. Some causes of contact dermatitis--such as poison ivy, oak and sumac--can cause contact dermatitis in most people. Other cases of contact dermatitis are caused by allergies and may not affect everyone. Examples of allergens that can cause rashes include nickel, some cosmetics and perfumes, and costume jewelry.
Autoimmune
Some rashes are the result of an abnormally functioning immune system. Patients with psoriasis or eczema, for example, frequently develop a rash as a result of these autoimmune diseases, Medline explains. Autoimmune diseases describe conditions in which the patient's immune system inadvertently attacks healthy tissue. Autoimmune diseases that cause rashes are a result of the immune system attacking skin cells, leading to inflammation that causes an itchy or scaly rash.
Infections
A variety of infections can cause a rash. Medline explains that many childhood diseases, including scarlet fever, mumps, measles, rubella and fifth disease can all lead to skin inflammation. Impetigo is a bacterial infection of the skin that can cause a rash and blisters. Shingles, which is caused by a reactivation of the virus that causes chicken pox, also commonly causes rashes. The Merck Manuals explains that sometimes the body has an unusual reaction to the herpes simplex virus, leading to a rash called erythema multiforme. Some fungal infections (such as ringworm, caused by the Tinea fungus) also cause rashes.
Allergies
Food and drug allergies can also lead to a rash. These rashes occur after a patient has ingested a food or drug. Some medications--such as tetracycline, chlorothiazide and sulfa antibiotics--cause the skin to become abnormally sensitive to sunlight. As a result, patients who take these medications can, according to the Merck Manuals, develop rashes after sun exposure.
