Many small bumps on the back of the arms may be caused by acne, keratosis pilaris, Malassezia folliculitis, and allergic factors. 1. Acne: It is related to the exuberant secretion of androgen in the body, increased secretion of hair follicles, and the propagation of Propionibacterium acnes in hair follicles, which may lead to a lot of small bumps on the back of the arms. 2. Keratosis pilaris: the pathogenesis is not clear, may be related to genetic factors, vitamin A deficiency, metabolic disorders, may cause skin roughness, skin follicular pimples with the size of a needle tip, can also cause a lot of small bumps on the back of the arm. 3. Malassezia folliculitis: Malassezia grows and multiplies in the hair follicle, and the stimulation of secretion will cause the hair follicle to expand and rupture, resulting in an inflammatory reaction, which will also lead to a lot of small bumps on the back of the arm. 4. Allergic factors: such as contact dermatitis, drug rash, eczema, etc. can also cause a lot of bumps on the back of the arms, accompanied by itching and other symptoms. There may be other reasons for the many bumps on the back of the arms, it is recommended to go to the hospital in time for examination and treatment as prescribed by the doctor.