Above figure (A) shows normal red blood cells flowing through veins.
Lower Figure (B) shows abnormal, sickled red blood cells piling up in a vein.

Sickle cell anaemia is a genetic disease. It affects red blood cells. It changes the cells from flexible disks into rigid crescents. When many red cells take this shape veins get blocked. This can cause damage to many organs. The organ damage increases with time and leads to an early death.