Red cell aging and destruction

The average life span of circulating erythrocytes is about 120 days. Aging of the red cells is associated with a decrease of the enzymatic activities, a progressive increase of hemoglobin concentration and red cell viscosity due to loss of total cation and water content ('dense red cells'), and structural modifications of the membrane with a reduction of the sur-

face:volume ratio, leading to the formation of a rigid spherocyte with enhanced osmotic fragility. These aspects occur in physiologic conditions, but similar features may be observed in various pathologic disorders associated with hemolytic processes.

Trapping of the senescent erythrocytes by reticuloendothelial cells is the last event of the life of these cells. This phenomenon may be viewed as an immunologic process characterized by the exposure of hidden antigenic sites followed by the binding of naturally occurring autologous antibodies (immunoglobulin G) which trigger the phagocytosis. Exposure of epitopes of band 3 may represent one of the events closely associated with the senescence of the erythrocytes.

Several other damaging processes (oxidative, proteolytic) may intervene during the life of the circulating erythrocytes; some of these are operative concomitantly with the presence of molecular defects of the cytoskeleton or of the hemoglobin structure, accelerating the red cell clearance.

See also: ABO blood group system; Agglutination; Anemia, autoimmune hemolytic in human; Blood transfusion reactions; Bone marrow and hematopo-iesis; Carbohydrate antigens; Complement receptors; Embryonic antigens; Hemolytic disease of the newborn; Protein separation techniques; Rh antigens; Spleen.

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