Hemoglobin which is also written as HB or HGB is a protein molecule present in red blood cells that takes oxygen from the lungs to the body's tissues and returns carbon dioxide from the tissues back to the lungs. Hemoglobin consists of four protein molecules (globulin chains) that are linked together. The normal adult hemoglobin molecule has two alpha-globulin chains and two beta-globulin chains. In fetuses and infants, beta chains are not frequent and the hemoglobin molecule is made up of two alpha chains and two gamma chains. As the infant grows, the gamma chains are steadily substituted by beta chains resulting in the adult hemoglobin structure.