Red blood cell hemoglobin is used to transport oxygen from the lungs to the tissues. It consists of two alpha chains and two beta chains. Normal adult hemoglobin (Hb A) is essentially composed of 95% to 98% Hb A and 2% to 3% Hb A2 with traces of minor hemoglobins (Hb A1, etc.). Blood also contains 1% to 2% fetal hemoglobin (Hb F) consisting of two alpha chains and two gamma chains. Hb F (hemoglobin F) is the normal hemoglobin of the fetus during pregnancy. Its production drops early after birth and reaches levels found in adults within 1 to 2 years. When abnormalities occur in normal Hb A production (thalassemia), there is often an increase in minor forms of hemoglobin (Hb A1 and Hb F).
Abnormally high levels of Hb F reaching up to 10% are found in carriers of the sickle cell trait (Hb S). Slightly elevated levels of Hb F are found in carriers of the beta thalassemia trait. In individuals who are homozygous for beta thalassemia (who have two copies of the mutated gene), hemoglobin F is the most abundant fraction of hemoglobin in the blood. Hemoglobin F significantly mitigates the effects of having a lack of normal hemoglobin.