Red patches on the body may be caused by urticaria, dermatomyositis, systemic lupus erythematosus and other factors. 1. Urticaria: it may be related to food allergy or infectious factors, which may lead to recurring reddish or pale reddish or irregular shapes of the skin, and the skin surface is uneven, which may be accompanied by itching and tingling, etc. Dermatomyositis: it may be related to genetic factors and environmental factors, which often appears as edematous purple patches. 2. Dermatomyositis: it may be related to genetic factors and environmental factors, often appearing edematous purplish-red patches, which are more frequent on the upper eyelids, forehead, cheeks and chest V area, with irregular edges, and may melt into flakes, and some of the patients may also suffer from fatigue, fever, loss of appetite and other symptoms. 3. Systemic lupus erythematosus: the cause of the disease is still unclear, may be related to genetic factors, viral infections, environmental factors, etc., may appear in the face, neck, back and other parts of the erythema, can also appear vasculitis rash, such as nodules, ulcers, etc., may be accompanied by joint pain, fever, fatigue, headache and other symptoms. Reddish patches on the body, or accompanied by other systemic symptoms, it is recommended to seek medical attention for detailed examination and treatment to avoid continued aggravation of the lesions.