Suddenly big bags on the body are itchy, which should be caused by allergies. After skin allergy, the most important manifestation on the skin is the appearance of big bags and itchy skin. External factors cause allergies, resulting in mast cell degranulation, release of histamine, manifested as wind masses and itching, the main causes of common allergies are the following: 1, food allergies, such as eating fish, shrimp, peanuts, apples, peaches, etc.; 2, oral medications cause allergies, such as eating cephalexin tablets, cotrimoxazole; 3, environmental factors caused by allergies, environmental humidity, environmental dust is more; 4, other causes caused by allergies, such as ultraviolet radiation, body with pathogenic bacteria infection, sports, etc.