CD59 (PNH Marker)

CD59 (protectin) is a glycosylphosphatidylinositol‑anchored membrane protein that inhibits formation of the membrane attack complex, protecting host cells from complement‑mediated lysis. It is widely expressed on erythrocytes, leukocytes and endothelial cells and is used clinically as a surface marker in flow cytometry. Loss or deficiency of CD59 on red cells is diagnostic for paroxysmal nocturnal hemoglobinuria.

Key markers for paroxysmal nocturnal hemoglobinuria (PNH) include loss or reduction of GPI‑anchored proteins—especially CD55 and CD59—on red cells, granulocytes and monocytes by flow cytometry. Laboratory findings: hemoglobinuria/urinary hemosiderin, elevated LDH, low haptoglobin, indirect hyperbilirubinemia, reticulocytosis, and cytopenias. There is also increased thrombotic risk with raised D‑dimer.

CD55 (decay-accelerating factor) protects blood cells from complement by accelerating decay of C3/C5 convertases on cell surfaces. In PNH, GPI-anchor deficiency causes loss of CD55 (and CD59) on red cells, permitting uncontrolled complement activation and membrane attack complex formation. This leads to intravascular hemolysis, hemoglobinuria, and increased risk of thrombosis and other PNH complications.

CD55 (decay-accelerating factor) and CD59 (protectin) are membrane proteins that protect host cells from complement-mediated damage. CD55 disrupts C3/C5 convertases to limit opsonization and downstream complement activation. CD59 blocks terminal complement complex assembly by preventing C9 polymerization, stopping membrane attack complex formation. Loss of these proteins (e.g., in paroxysmal nocturnal hemoglobinuria) causes complement-driven red cell lysis.

The hallmark of paroxysmal nocturnal hemoglobinuria (PNH) is complement‑mediated intravascular hemolysis due to acquired PIGA gene mutations that cause loss of GPI‑anchored proteins, chiefly CD55 and CD59, on red blood cells. This renders RBCs hypersensitive to complement, causing hemoglobinuria (often worse at night), hemolytic anemia, and increased risk of thrombosis and bone marrow failure.

CD59 is also known as protectin and as the membrane attack complex–inhibitory protein (MAC‑IP). It is often called CD59 glycoprotein or CD59 antigen. This small, GPI‑anchored complement regulatory protein prevents formation of the membrane attack complex on host cells, protecting them from complement‑mediated lysis. It is expressed widely on red blood cells, leukocytes, endothelial cells and many other tissues.