Specificity: Anti-CD31 has shown to be highly specific and sensitive for vascular endothelial cells. Staining of nonvascular tumors (excluding hematopoietic neoplasms) is rare. Anti-CD31 reacts with normal, benign, and malignant endothelial cells which make up blood vessel lining. Immunogen: Membrane preparation of a spleen from a patient with hairy cell leukemia.