Why do red cells sickle?
There is a substance (a protein) in the red cell called Hemoglobin (Hb) that carries oxygen inside the cell. One little change in this substance causes it to form long, rigid rods in the red cell when there is not enough oxygen, changing the round red cell into a sickle shape. These changes are pre-defined by your genes before you are born and can be predicted by tests on the unborn fetus while still in the womb.
