Blood type is a classification system used to determine which blood is compatible for a transfusion, preventing a potentially fatal immune reaction. Classification is based on the presence or absence of specific protein markers, known as antigens, found on the surface of red blood cells. The two most significant systems used in medicine are the ABO group and the Rh factor, which combine to create the eight common blood types. AB positive blood, or AB+, is one of these types.
The Biological Markers of AB Positive Blood
The ABO system classifies blood into four main groups—A, B, AB, and O—depending on which antigens are present on the red blood cell surface. Individuals with AB blood possess both the A antigen and the B antigen on their red blood cells. This dual presence is genetically determined and is the defining characteristic of the AB group.
The second component, the “positive” (+) sign in AB+, refers to the presence of the Rh factor, specifically the D antigen. Since AB+ individuals have the D antigen, they are classified as Rh-positive. The immune system develops antibodies against any antigens that are not present on a person’s own red blood cells. Because AB+ red blood cells display all three major antigens—A, B, and Rh (D)—the plasma of an AB+ individual does not produce antibodies against any of these markers.
The Universal Recipient: AB+ in Transfusions
The absence of anti-A and anti-B antibodies in the AB+ bloodstream is what grants this blood type its designation as the “universal recipient” for red blood cell transfusions. Since the recipient’s immune system has no antibodies to attack the A, B, or Rh antigens, an AB+ patient can safely receive red blood cells from any of the eight major blood types. This compatibility makes AB+ patients highly flexible in emergency situations where blood type matching may need to be done rapidly.
Despite being a universal recipient for red blood cells, AB+ individuals can only donate red blood cells to other people who also have AB+ blood. This is because their red blood cells carry both A and B antigens, which would trigger a severe immune reaction in any patient with A, B, or O blood, as those types all have corresponding antibodies in their plasma. Therefore, AB+ red blood cells are highly specific in their donation potential.
Plasma Donation
The role of AB blood in plasma donation is the opposite of its red blood cell compatibility. Plasma is the liquid component of blood that contains antibodies. Since AB individuals produce neither anti-A nor anti-B antibodies, their plasma can be safely transfused to patients of any ABO blood type. This makes AB plasma the “universal donor” for plasma transfusions, a component often used to treat burn victims and trauma patients.
Genetics and Global Prevalence
The AB blood type is genetically inherited, requiring a person to inherit an A allele from one parent and a B allele from the other. The A and B alleles are codominant, meaning that when both are present, they are both fully expressed, resulting in the AB blood type. The Rh factor is inherited separately, where having at least one gene for the D antigen results in the positive classification.
The AB+ blood type is one of the less common blood types globally, often making it a valuable resource for blood banks. In the United States, for example, AB+ blood is found in approximately 3.4% of the population. The AB group overall, including both positive and negative Rh factors, remains the rarest of the four ABO types.