Most of the bruises on the legs are due to trauma, abnormal coagulation function, allergic reaction, etc. The cause of the bruises is not the same as the cause of the allergic reaction. As each person’s personal situation is different, it is recommended to go to the hospital for examination to clarify the cause of the disease and treat the cause. 1. Trauma: When the leg is traumatized, the capillaries under the skin rupture and bleed, resulting in subcutaneous bruising and bruising. 2. Abnormal coagulation function: when people suffer from blood diseases (such as hemophilia, thrombocytopenic purpura, etc.), the coagulation function is abnormal, and the skin is prone to bruises and purple patches. It is recommended that treatment be carried out under the guidance of a doctor. 3. Allergy: When people suffer from allergic purpura, the blood vessels will undergo inflammatory reaction, and the legs will have the uncomfortable symptoms of bruises and purple patches.