Hemoglobin

In biology, Hemoglobin is the "oxygen-carrying pigments of erythrocytes. They are found in all vertebrates and some invertebrates. The structure of the globin moiety differs between species."

Fetuses
In human fetuses the normal hemoglobin is Hemoglobin F (also called Hemoglobin F). Hemoglobin F has "two alpha and two gamma polypeptide subunits in comparison to normal adult hemoglobin, which has two alpha and two beta polypeptide subunits. Fetal hemoglobin concentrations can be elevated (usually above 0.5%) in children and adults affected by leukemia and several types of anemia."

Adults
The normal adult human hemoglobin is Hemoglobin A. This hemoglobin consists of two alpha and two beta chains. A less common hemoglobin found in normal adults is Hemoglobin A2. Hemoglobin A2 is an "adult hemoglobin component normally present in hemolysates from human erythrocytes in concentrations of about 3%. The hemoglobin is composed of two alpha chains and two delta chains. The percentage of HbA2 varies in some hematologic disorders, but is about double in beta-thalassemia."