The causes of reticular red marks on the arm include atopic dermatitis, capillary dilatation, lymphangitis, superficial phlebitis, etc. 1. Allergic dermatitis: because the skin is allergic to certain substances, after contact with the sensitizer, then there will be an allergic reaction triggered by vasodilatation, so there is the appearance of reticular red marks. 2. Capillary dilatation: when various factors lead to capillary dilatation, it will also lead to the appearance of red reticular marks on the arm. 3. Lymphangitis: when pathogenic bacteria infect the lymphatic vessels, inflammation of the lymphatic vessels can occur, and the phenomenon of red reticular marks can be seen. 4. Superficial phlebitis: the etiology is complex, may be related to obesity, bacterial infection, can lead to redness and swelling of the lesion area, can also cause reticular red marks on the arm. Reticulated red marks on the arm can also have other reasons, it is recommended that timely to the hospital for examination, standardized treatment.