A string of bumps on the arm when you scratch it is mostly seen in allergies, such as artificial urticaria, pruritus, etc., which may cause the above symptoms. 1. Artificial urticaria: artificial urticaria is caused by various causes of mast cells and other inflammatory cell activation and degranulation, the release of inflammatory activity of chemical mediators leading to skin lesions, the clinical manifestations of scratching or scratching with a blunt instrument across the skin for a few minutes along the scratches appeared as stripes, with or without itching, about half an hour after the spontaneous subsidence. 2. Pruritus: Pruritus is usually secondary to other diseases, such as eczema, mosquito bites, medications and so on. Mosquito bites cause itching and scratching with the hands, especially in children, which can lead to a series of localized bumps with obvious itching. Frequent occurrence of the above symptoms, it is recommended to go to the hospital dermatology department, under the guidance of the doctor standardized treatment, can help relieve the symptoms.