Under low oxygen tension, homozygous hemoglobin S will polymerize, forming tactoids or fluid polymers. These polymers realign and cause the red blood cell to become deformed. While reoxygenation reverses the deformed shape, it is only temporary as the tactoids continue to grow during repeated passes through the sickling cycle as shown in image 1.
This process is enhanced by a low pH, increased levels of 2,3-DPG, increased temperature (above 37oC), and a state of dehydration. As the cycle is repeated, a potential for occlusion exists as cells become more rigid and contribute to increased viscosity (see image 2).
Repeated occurrence of sickling eventually leads to an irreversibly-damaged cell, which is involved in occlusion or otherwise removed from circulation (see image 3).