Hemoglobin is the protein in red blood cells that carries oxygen from the lungs throughout the body. In each red blood cell, there are hundreds of hemoglobin molecules. Hemoglobin molecules are made of two subunits, alpha and beta. Both the alpha and beta subunits are needed for hemoglobin to function correctly. When there is a change in the way hemoglobin is made, the red blood cells cannot deliver oxygen as efficiently.