The skin is extremely important for protecting your body from infection, controlling your body temperature and helping you to sense pain. Because your skin covers your body and is in connection with the rest of your tissues through lymph channels and nerve cells, it is a common site of response to illness. The body responds to infection or illness by changing the skin's color and texture in various locations, including red blotches that can indicate an underlying concern.
If you have had a recent injury, including a small cut or scrape, you can develop a bacterial infection that can spread on the skin of your legs. This is known as cellulitis and occurs when bacteria infect the tissue beneath your skin; causing redness and swelling. According the University of Maryland Medical Center, the condition may occur anywhere, but most commonly affects the legs. Because it is caused by a bacterial infection, cellulitis is commonly treated with antibiotics to reduce redness and control further spread.
Red blotches under the skin on your legs may be caused by an allergic reaction. When you are allergic to something in the environment, your body protects itself by releasing substances into the bloodstream. These substances often affect the skin and may produce red blotches on the body, including your legs. The blotches on your legs may be raised or itchy. People are allergic to various items in the environment, including pollen, some types of medication, detergents and foods. If you think you have been exposed to an allergen, your skin may be reacting by producing red blotches.
Erythema multiforme is a condition that causes patches of red skin that may be raised or itchy. The condition occurs as a result of the herpes simplex virus and may also be associated with cold sores or a fever. It can also be caused as a reaction to medication or a recent immunization. Red, blotchy skin may last for approximately two to four weeks before resolving on its own; although the condition may be ongoing and reddened skin may recur.
Some people develop red skin on the legs due to an autoimmune disorder. This type of disorder occurs when the body recognizes its own parts as foreign and attacks the offending tissue. This produces varied results, but may lead to pain or skin problems, depending on the type of disorder. For example, the autoimmune disease lupus produces red lesions that may worsen with exposure to sunlight. Lupus is also associated with other symptoms, including mouth sores and muscle pain, so reddened skin may only be one manifestation of the disease.
Because many skin rashes and infections can mimic each other in appearance, it may be difficult to determine the exact cause of red, blotchy skin on your legs. If you have other symptoms; including pain, fever or joint swelling; or if your skin is also swollen, itchy or oozing fluid, contact your doctor for treatment as these can be signs of a larger infection.