Blood typing classifies blood based on specific protein markers, known as antigens, found on the surface of red blood cells. The AB positive (AB+) designation is one of the eight most common blood types, derived from the ABO system and the Rh system. Understanding these surface markers explains the distinct role AB+ blood plays in medical transfusions.
The Biology of AB Positive Blood
The AB+ blood type is defined by a specific set of antigens on the red blood cell surface and a corresponding lack of antibodies in the plasma. Under the ABO system, AB blood means the red blood cells possess both the A antigen and the B antigen.
The “positive” aspect of AB+ is determined by the Rhesus (Rh) factor, specifically the presence of the D antigen. Therefore, AB+ blood cells carry three distinct antigens: A, B, and Rh (D).
The plasma of an AB+ individual contains neither Anti-A nor Anti-B antibodies. Antibodies are immune proteins that recognize and attack foreign antigens. Since AB+ red blood cells carry both A and B antigens, the immune system does not produce antibodies against them. This absence of ABO antibodies dictates its role in transfusions.
Transfusion Role: The Universal Recipient Status
The lack of Anti-A and Anti-B antibodies in the plasma makes AB+ individuals the “Universal Recipient” for red blood cell transfusions. Compatibility is primarily determined by whether the recipient’s antibodies will attack the donor’s red blood cell antigens. An AB+ recipient’s plasma will not react against A antigens, B antigens, or a combination of both.
This means that AB+ patients can safely receive packed red blood cells from donors of any ABO type—A, B, O, or AB. Since the blood is also Rh positive, they can receive red cells that are either Rh-positive or Rh-negative. This wide compatibility range is particularly valuable in emergency situations when a patient’s blood type is unknown and there is no time for full cross-matching procedures.
Unique Value in Plasma and Platelet Donation
While AB+ individuals are universal recipients for red blood cells, their plasma is considered the “Universal Plasma Donor.” Plasma compatibility follows the inverse rule of red blood cell compatibility, focusing on the antibodies in the donor’s plasma. Because AB+ plasma lacks both Anti-A and Anti-B antibodies, it can be safely given to patients of any ABO blood type.
If plasma containing Anti-A or Anti-B antibodies were given to an incompatible recipient, those antibodies could attack the patient’s red blood cells. The antibody-free nature of AB+ plasma removes this risk, making it an extremely versatile product. This plasma is frequently used in trauma, burn, and clotting factor deficiency cases where rapid and universal compatibility is necessary. AB+ donors are also encouraged to donate platelets, which are highly valuable for patients with cancer or those undergoing major surgery.
Rarity and Genetic Inheritance
The AB+ blood type is one of the rarest major blood types across the globe. In the United States, it is estimated that only about 3% to 4% of the population has AB+ blood. Its rarity is due to the genetic inheritance pattern required to express both the A and B antigens, along with the Rh factor.
An individual inherits their ABO blood type from alleles from both parents. For a child to have type AB blood, they must inherit an A allele from one parent and a B allele from the other, as the A and B alleles are co-dominant. Possible parent pairings include A and B, A and AB, B and AB, or AB and AB. The Rh positive factor is inherited separately, requiring the presence of the dominant RhD gene from at least one parent. This specific combination of required alleles contributes to the lower prevalence of AB+.