Red patches on the legs can be caused by allergic factors, and if they are allergic, they are usually seen in urticaria or papular urticaria, also known as insect bite dermatitis. Of course, there are also other causes of allergic dermatitis and contact dermatitis, as well as contact dermatitis caused by medications. In addition to allergic factors, it may also be cutaneous vasculitis, as cutaneous vasculitis occurs mostly on the legs, especially on the lower legs, and may appear as limited or scattered red patches, and may be accompanied by varying degrees of pain or itching. If the inflammatory response is significant, some plaques may be accompanied by varying degrees of exudation or ulceration.