Small bumps on the inside of the arm are considered to be related to factors such as folliculitis, skin allergies, and perifollicular keratosis. 1. Folliculitis: Folliculitis occurs in relation to bacterial infection, where a small injury to the skin occurs and pathogenic bacteria take advantage of the situation, leading to inflammation of the hair follicle and its surroundings. Single or multiple red papules may appear locally, usually accompanied by redness, swelling and pain. 2. Skin allergy: Skin allergy is related to allergic constitution, such as contacting, inhaling or ingesting substances that can easily trigger allergy, resulting in allergic urticaria, small bumps may appear on the skin, which may be accompanied by itching. 3. Keratosis Pilaris: Keratosis Pilaris is considered to be genetically related and can occur on the patient’s limbs, such as the outer side of the arms, and may appear as isolated papules accompanied by raised bumps. In addition, lipomas and sebaceous cysts can also cause bumps on the skin. It is recommended that patients go to the hospital in time for examination to understand the causes of the skin bumps and target treatment.