Immunohistochemistry for CD31 (also known as PECAM-1) aims to detect a specialized membrane protein highly expressed in junctions between vascular and lymphatic endothelial cells and in some blood cells. It is a highly sensitive and specific marker for identifying the presence and distribution of endothelial cells, making it an essential tool in assessing microvessel density (Angiogenesis) and in diagnosing vascular tumors.