Leukocyte Extravasation: Leukocyte (loo´ko-sīt) or white blood cell is one of the three major blood components, along with red blood cells (erythrocytes) and platelets (thrombocytes). Each of these plays a vital role in maintaining homeostasis in the body. These leukocytes are freely floating in the blood stream. For instance, if you happen to sustain an injury which damages the underlying tissues, this is the time that leukocytes would attach into the tissues to repair the trauma and this mechanism is called leukocyte extravasation.
Extravasation can only take place in post-capillary venules because this is the specific area wherein there is minimal hemodynamic shear force. Same thing also happens when you have an ongoing infectious process. However, leukocyte extravasation only takes place under special circumstances. For this to occur there must be communication between leukocytic cells and the endothileal cells which compose the blood vessel lining. As part of the ongoing inflammatory process, the endothileal cells would exocytose a special chemical called P-selectin which would eventually lead to the synthesis of platelet aggregating factor (PAF). Normally, there are oligosaccharides on the surfaces of the leukocytes which bind with P-selectin. The bond formed is weak. Thus, this has to roll along the endothileal cells. PAF receptors and integrin are also found in the leukocytes. As the PAFs bind with their receptors integrins would then be activated. On the other hand, ICAM molecules can be found along the surface of the endothileal cells. The activated integrins in the leukocytes can then strongly adhere to the endothileal lining. These leukocytes would then travel from one endothileal cell to another to eventually reach the damaged site. The adhered leukocytes can then create a sturdy attachment to the site of injury. This is practically how leukocyte extravasation takes place.