Pimples on the arm, can be seen in insect bite dermatitis, follicular keratosis or chicken pox and other diseases. 1. Insect bite dermatitis: in summer, mosquitoes can sting the exposed skin to suck blood, which can cause a stinging sensation, some people do not have obvious rash after stinging, but there can be erythema, papules and other lesions accompanied by obvious itching, scratching aggravates the lesions, so that the range of lesions can be increased. 2. Follicular keratosis: belongs to the autosomal dominant inheritance caused by abnormal keratinization of the skin disease, can occur in the whole trunk, limbs flexion and other parts. At first, the lesions are firm pinpoint to mung bean-sized papules with normal skin color, but then the top of the papules are crusted and become dark brown, and sometimes can be fused. 3. Chickenpox: When a person is infected with human herpes virus type III, varicella zoster virus, the virus spreads to the epidermal keratin forming epithelial cells and mucosal epithelial cells, which can cause a red rash all over the body, such as the head, face, trunk, limbs, etc., and then the rash can change into papules, papules or blisters. If the arm acne is more recommended timely medical care, clear nature of the lesions, follow the doctor’s instructions reasonable medication.