What is the pathogenesis of paroxysmal nocturnal hemoglobinuria (PNH)?

Updated: May 20, 2021
  • Author: Emmanuel C Besa, MD; Chief Editor: Sara J Grethlein, MD, FACP  more...
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Answer

When intravascular hemolysis occurs, RBCs release hemoglobin (Hb) into the plasma. This free Hb is rapidly dimerized and bound by serum protein haptoglobin and rapidly removed by macrophages, which then degrades it after endocytosis. Since haptoglobin is not recycled, large amounts of free Hb can deplete the body's supply, leaving the excess Hb free in the plasma.

When the capacity to manage and degrade free Hb during acute or chronic hemolysis is reached, levels of Hb and heme increase in the plasma and urine. Plasma Hb has the ability to scavenge nitric oxide (NO), resulting in rapid consumption of NO and clinical sequelae of NO depletion. NO plays a major role in vascular homeostasis and has been shown to be a critical regulator of basal and stress-mediated smooth muscle relaxation and vasomotor tone, endothelial adhesion, and platelet activation and aggregation.


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