diapedesis
Summary
Diapedesis is the process by which leukocytes squeeze through the endothelial cell junctions of postcapillary venules to migrate from the bloodstream into surrounding tissue, typically in response to infection or injury. It is a key step in the inflammatory response, occurring after margination, rolling, and firm adhesion of leukocytes to endothelium.
Detail
Diapedesis (also called transmigration or extravasation) is part of the leukocyte adhesion cascade during acute inflammation. The sequence begins with margination (leukocytes move to the periphery of blood flow), followed by rolling (mediated by selectins binding sialyl-Lewis X), then firm adhesion (mediated by integrins like LFA-1 binding ICAM-1), and finally transmigration (diapedesis) through the endothelial junctions, primarily driven by PECAM-1 (CD31) interactions between leukocytes and endothelial cells. Once in the extravascular tissue, leukocytes follow a chemotactic gradient (e.g., C5a, IL-8, LTB4, bacterial products) to reach the site of injury or infection. Clinically, defects in this process are seen in Leukocyte Adhesion Deficiency (LAD) type 1, caused by a defect in CD18 (integrin beta-2 subunit), leading to impaired firm adhesion and diapedesis, resulting in recurrent bacterial infections, delayed separation of the umbilical cord, and absence of pus formation despite high neutrophil counts. Understanding diapedesis is essential for understanding acute inflammation, immune cell trafficking, and the pathophysiology of various immunodeficiencies and inflammatory diseases.
Sources
- Robbins and Cotran Pathologic Basis of Disease
- First Aid for the USMLE Step 1
- Kumar, Abbas, Aster - Robbins Basic Pathology
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