There is no collision but bruises on legs are often caused by allergic reaction and coagulation dysfunction, it is recommended that patients go to the hospital in time, under the guidance of the doctor for blood routine, coagulation items and other related examinations to clarify the cause of the disease and then treat the cause. I. Allergic reaction: For example, bruises or dark purple patches may appear on the skin and mucous membranes of patients with allergic purpura, including the legs, which may also usually be accompanied by rashes and joint swelling and pain. Second, coagulation dysfunction: 1, blood system diseases: for example, thrombocytopenic purpura patients skin, mucous membrane bleeding, can be manifested as bruises or bruises on the legs, may also appear increased menstruation, nasal bleeding, serious cases may even appear visceral bleeding; leukemia patients legs can appear subcutaneous bleeding, and then can form bruises, while may be accompanied by pale, fever, anemia and other symptoms; aplastic Patients with aplastic anemia may have bruises on the legs, nosebleeds, etc. Women may also bleed, have excessive menstruation, prolonged periods, etc. Serious cases may even occur intracranial hemorrhage; patients with hemophilia often have slow blood seepage from joints, muscles, skin, mucous membranes, etc., which may lead to bruises on the legs; 2, liver diseases: for example, viral hepatitis, cirrhosis of the liver, etc. can lead to the body coagulation 2, liver disease: such as viral hepatitis, liver cirrhosis, etc. can lead to the body’s blood clotting disorder, which leads to the leg subcutaneous capillary brittleness increases, rupture bleeding, and bruising.