Itchy small bags on legs and arms may be due to insect bite dermatitis, urticaria, acute eczema, folliculitis and other factors. 1. Insect bite dermatitis: local skin allergic and inflammatory reactions occurring after the bite of mosquitoes, fleas and other insects, common in summer and fall seasons, mostly in the arms, legs and other exposed parts of the body, manifested as sudden bright red, windswept pimples, the size of a pea to a fingertip, itchy and obvious, usually within a few hours to improve. 2. Urticaria: it can be spontaneous, or due to food, drugs, sunlight, scratches and other factors, can be manifested as white or bright red lumpy bumps, varying in size, can be accompanied by itching, exogenous urticaria can disappear on its own. 3. Acute eczema: it is caused by a combination of internal and external factors, easy to recur, can be manifested as corn grain-sized papules, blisters, papules, can be fused into a piece, the border is not clear, itching is intense. 4. Folliculitis: most of the folliculitis is due to pathogens caused by hair follicle infection, manifested in the formation of red papules around the hair follicle, and accompanied by red, swollen, hot, painful inflammatory reaction, the patient may be conscious of the itching or pain, and in severe cases, pus may also be present, broken. Itchy small bags on the legs and arms may also be due to other factors, it is recommended that patients seek medical attention in a timely manner, under the guidance of the doctor for relevant examinations, in order to clarify the cause of the disease, targeted treatment.