Itchy red bags on the arm may be insect bite dermatitis, urticaria, eczema and other diseases. 1. Insect bite dermatitis: a perverse reaction caused by insect bites, if the arm is bitten by an insect, it can be manifested as bright red windswept pimples, erythema, pimples, with itching. Scratching the affected area should be avoided to prevent secondary infection. 2. Urticaria: If patients eat, inhale or contact allergens in daily life, such as seafood, mango, pollen, animal hair, etc., urticaria can be induced, and edematous masses appear on the arms, and the lesions are of different sizes, and if they often scratch, it will lead to a continuous increase in the number of lesions. 3. Eczema: the cause is unknown, is internal and external multi-factorial inflammatory skin disease, may be related to heredity, infection, allergy, immunity, mental overstress and other factors, can appear on the arm erythema, pimples, papules, etc., accompanied by intense itching symptoms, and in severe cases, oozing, vesiculation. Itchy red bags on the arm may also be caused by other diseases, it is recommended that patients go to the regular hospital in a timely manner, after a clear diagnosis under the guidance of the doctor to give targeted treatment.