Pathobiology of sickle cells
The sickle shape derives from the tendency for Haemoglobin S to polymerise into a linear form when oxygen tension is low. This polymerisation causes the characteristic distortion of the cell. The sickling process is initially reversible, but secondary damage to the red cell caused by repeated sickling causes intracellular dehydration and damage. This eventually results in cells with permanent sickle morphology accompanied by other damaged or abnormal cellular forms, particularly target cells, boat-shaped forms and contracted cells. On blood films the reversibly sickled cells rapidly revert to normal morphology during preparation so only irreversibly sickled cells will be detected.