Emigration

EMIGRATION, the process of formed elements of blood exiting the vessel lumen. Emigration occurs most frequently with polymorphonuclear leukocytes (mainly neutrophils); however, E. is also observed in other elements: lymphocytes, monocytes, erythrocytes. In the latter case, it is more common to speak of diapedesis (see). Emigration is observed mainly in inflammation (see); it is also noted in certain physiological processes, for example, in the gastrointestinal tract during digestion (so-called leukodiapedesis). The essence of the emigration process is not fully understood. Some assume a purely biological essence of the phenomenon, based on the chemotaxis of leukocytes. Others see in E. a physicochemical process. Sometimes instead of E., the term immigration is used. Immigration, a term sometimes used in embryology and pathology to denote the entry of cellular elements located outside of any hollow tube (for example, a vessel, excretory duct) into the latter; thus, the concept of immigration is opposed to E. Immigration is observed as a physiological process in hematopoiesis (for example, the formation of 'histiocytes of blood', monocytes from reticulo-endothelium), in the development of various tissues and organs from the embryonic leaflets of the fetus, and in pathology mainly in inflammation, when the separating and multiplying cells of the endothelium and adventitia of blood vessels can actively or passively penetrate into the blood flow; inflammatory immigration does not, however, exclude the possibility of subsequent E. of the same elements. Immigration into glandular formations (for example, the penetration of leukocytes into renal tubules in interstitial nephritis) can cause changes in the secretion of the corresponding gland. Immigration of free cells into compact tissue masses is more commonly designated as cellular infiltration.