A person with sickle cell anemia has an inherited disease that causes the red blood cells to have an abnormal shape. As a result, the red blood cells function poorly and form small clots in the bloodstream. When the small clots form in the blood vessels, the person experiences severe pain, called a painful crisis.

A person with sickle cell trait carries some of the genes for sickle cell, but does not have sickle cell anemia.