Sickle cell disease cannot be caught, acquired, or otherwise transmitted. Also, sickle cell trait does not develop into sickle cell disease. Ordinarily, the hemoglobin molecules exist as single, isolated units in the red cell, whether they have oxygen bound or not. Normal red cells maintain a basic disc shape, whether they are transporting oxygen or not. The picture is different with sickle hemoglobin.
Sickle hemoglobin exists as isolated units in the red cells when they have oxygen bound.
When sickle hemoglobin releases oxygen in the peripheral tissues, however, the molecules tend to stick together and form long chains or polymers. These rigid polymers distort the cell and cause it to bend out of shape. While most distorted cells are simply shaped irregularly, a few have a cresent-like appearence under the microscope. A single red cell may traverse the circulation four times in one minute. Sickle hemoglobin undergoes repeated episodes of polymerization and depolymerization.