When local inflammation is caused by trauma, infection, or other conditions in the hand or arm, the lymphatic vessels that may be invaded by the bacteria causing the inflammation. Since lymphatic fluid flows along the lymphatic fluid from the distal end to the proximal end, bacteria also move along the lymphatic vessels from the distal end to the proximal end, and the lymphatic vessels invaded by bacteria show stiff streaks of red, and the infected limb shows inflammatory symptoms such as redness, swelling, heat, and pain approximately locally. In conclusion, red lines on the arms are usually caused by lymphangitis due to bacterial invasion of the lymphatic vessels, which needs to be treated promptly to prevent the spread of inflammation and serious systemic reactions.