Purpura refers to the color change of skin and mucosa after blood. The clinical manifestations are bleeding spots, purpura and ecchymosis, which are generally not higher than the skin surface. Only when allergic purpura occurs, it can be slightly raised, starting with purple, not fading after being pressed, then gradually becoming shallow, and turning yellow and fading after about two weeks.

(1) The etiology of simple purpura is unclear, generally mild, with no obvious changes in blood vessels and occasional slight coagulation dysfunction.

(2) Henoch-Schonlein purpura often occurs due to the increase of permeability and brittleness of blood vessel wall, and the vascular system changes, but the blood itself does not change. Therefore, the number of platelets, coagulation time and prothrombin time are all in the normal range. It is a vascular allergic disease, because the body reacts to some allergens, which leads to the increase of capillary fragility and permeability, blood extravasation, skin, mucosa and some organs bleeding and other allergic manifestations.

(3) Idiopathic thrombocytopenic purpura, also known as hemorrhagic purpura, the cause of which is unknown. However, antiplatelet factors can be found in the blood of some patients, which can reduce platelets and lead to bleeding. It is a hemorrhagic disease in which platelet immunity is destroyed and platelets in circulating blood are reduced. It is characterized by extensive bleeding of skin mucosa and organs, thrombocytopenia, normal or increased number of megakaryocytes in bone marrow with maturity disorder, shortened platelet life and positive anti-platelet antibody. More common in children and young adults, women are more than men.