Red blood cells are normally shaped like doughnuts, but sickle cells (purple) are flattened and clump together. NIH hide caption

itoggle caption NIH

Nurse Corean McClinton, left, talks about pain management with Sherry Webb at the Sickle Cell Disease Center in the Truman Medical Center, in Kansas City, Mo., in 2007. Dick Whipple/Associated Press hide caption

itoggle caption Dick Whipple/Associated Press