A large piece of red, swollen, raised and particularly itchy skin may be caused by insect bite dermatitis, skin allergies and other skin diseases. Insect bite dermatitis: after insect bites, the local skin will produce a perverse reaction, erythema, pimples, scratching can also form blisters, and with itching. Skin allergy: food, drugs, skin contact or some physical factors such as sunlight can cause patients with skin allergies, there can be air masses, erythema, papules, blisters and other lesions, accompanied by obvious itching. A large piece of skin red and swollen bumps especially itchy cause more, it is recommended to go to the hospital, follow the doctor’s instructions for treatment, so as not to affect the health.