Arms red packet itchy for no reason may be caused by contact dermatitis, hives, insect bite dermatitis and other causes, the specific causes also need to go to the hospital to determine the examination. 1.contact dermatitis: contact dermatitis may be caused by contact with exogenous sensitizing substances, the main exogenous substances are divided into animal, plant, chemical, such as caterpillars, lacquer, soap, dyes, etc., lesions are mostly clear boundaries of the erythema, the size of the tip of the needle dense papules, blisters, accompanied by pain, itching, burning, etc., the condition is serious, but also nausea, fever, headache, chills and other systemic symptoms. In severe cases, nausea, fever, headache, chills and other systemic symptoms may occur. 2. Urticaria: Urticaria is generally caused by viral infection, sun exposure, emotional fluctuations, etc., the patient’s skin will suddenly appear itching, itching will appear after the pale white or red air masses, some patients will also be accompanied by chest tightness, abdominal pain and other symptoms. 3. Insect bite dermatitis: insect bite dermatitis is generally associated with mosquito bites, when the arm is bitten by mosquitoes, the patient will appear in the early fusiform doughnut, with edema, with the development of the disease, the patient edematous doughnuts on the basis of large blisters may also appear, and accompanied by intense itching. If the patient appears arm red bags for no reason, itchy situation, while there are other accompanying symptoms, should be timely to the hospital for a clear diagnosis, and comply with the doctor’s instructions for treatment.