Updated: 2026 What is Sickle Cell Disease (SCD)? Sickle cell disease (SCD) is an inherited disorder of the red blood cells. Healthy red blood cells are round and donut-shaped. They move through blood vessels throughout the body to carry oxygen, carbon dioxide, and nutrients. Red blood cells rely on a special protein called hemoglobin to transport oxygen. In SCD, there is a genetic change that causes the hemoglobin, the main component of the red blood cells to stick together.