Hemoglobin

Structure of human hemoglobin.

Hemoglobin is a protein found in red blood cells that delivers oxygen from the lungs to other tissues. Interestingly, hemoglobin is identical among all humans. As blood types are formed by the difference in the membrane of the blood cells and the hemoglobin is inside the membrane, even if individuals have different blood types, their hemoglobin are identical.

Photo Credit: 
Richard Wheeler (Zephyris) on Wikimedia Commons