Membrane structure and its relation to haemolysis.

Advances in our understanding of the biochemistry, physiology and ultrastructure of the normal erythrocyte membrane have opened up the possibility of characterising congenital or acquired membrane defects in various haemolytic anaemias. The normal red cell membrane can undergo reversible disc-echinocytic or disc-stomatocytic shape changes in response to a wide variety of chemical agents and conditions. These shape changes may become irreversible and, in fact, they may be encountered in different types of haemolytic disease, suggesting that the echinocytic and stomatocytic shape changes represent two fundamental ways in which red cells react to intrinsic and extrinsic insults. Membrane alterations in haemolytic disease can be divided into disorders in which the defect appears to be a primary membrane abnormality, e.g. hereditary spherocytosis and disorders in which membrane damage occurs secondary to internal or external influences, e.g. the membrane alteration in sickled cells. The normal of deformability of the red cell is one of the prime determinants of survival in vivo and thus the geometric relationship of the erythrocyte surface area to cell volume, if decreased, can contribute to haemolysis. Intrinsic changes in membrane deformability or membrane deformability secondary to alterations in the cell haemoglobin may also compromise passage through the microcirculation, particularly through the spleen. Finally, abnormalities of cation permeability and/or lipid composition are found in certain haemolytic states. In each case it is essential to attempt to relate the abnormality described to the manner in which cell properties critical to in vivo survival are compromised in order to understand the pathophysiology.