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“Innate Immunity:

)THE CELLULAR RESPONSE This is characterized by extravasations of leucocytes from the lumen into interstitial tissue followed by phagocytosis.  Extravasations involve the following sequence of events: – a) Margination b) Transmigration across the endothelium to interstitical tissue (also called diapedesis). c) Migration in the interstitial tissues towards a chemotactic stimulus called Chemotaxis. a) Margination of leukocytes  It is the adherence of leukocytes to the endothelial cells lining. Mainly to the post Capillary venules. b) Transmigration of leukocytes  This is the movement of leukocytes by extending pseudopodia through the vascular wall by a process called diapedesis.  Leukocytes escape from venules and small veins but only occasionally from capillaries. c) Chemotaxis  It is a unidirectional leukocyte attraction within tissue space guided by the presence of bacteria and cellular debris.  All granulocytes, monocytes and to a lesser extent lymphocytes respond to chemotactic stimuli. d) Phagocytosis Pathophysiology 30  Once the cell has reached to the site of injurious agent (in interstitial tissue) phagocytosis ensues.  Phagocytic cells include polymorphonuclear leukocytes (particularly neutrophils), monocytes and tissue macrophages.  Phagocytosis involves three distinct but interrelated steps:- 1. Recognition and attachment of the particle to be ingested by the leukocytes:  Phagocytosis is enhanced if the material to be phagocyted is coated with certain plasma proteins called opsonins. 2. Engulfment  As a result of fusion between the p h a g o s o m e a n d l y s o s o m e , a phagolysosome is formed and the engulfed particle is exposed to the degradative lysosomal enzymes 3. Killing or degradation  The ultimate step in phagocytosis of bacteria (any foreign body) is killing and degradation.