Small red dots on the arms may be related to atopic dermatitis, thrombocytopenic purpura, folliculitis and other causes. 1. Allergic dermatitis: After allergens enter the body through skin or mucous membrane contact, injection and ingestion, they can lead to an abnormal immune response and the appearance of red pimples. 2. Thrombocytopenic purpura: Reduced number or abnormal function of platelets in the blood, lack of coagulation factors or abnormal function and other factors can cause the blood to leak out from the walls of small blood vessels, appearing as small red dots. 3. Folliculitis: Bacterial infection of the arm can cause localized inflammatory reaction of the hair follicle, which can appear as small red dots. It is recommended to go to the hospital in time to clarify the cause of the arm red dots, under the guidance of the doctor for standardized treatment.