Blood types classify human blood based on inherited substances on the surface of red blood cells. These classifications are primarily determined by the ABO and Rh blood group systems. Among the various blood types, AB positive (AB+) is recognized as the rarest, found in only a small percentage of the global population.
The Basics of Blood Types
The ABO blood group system categorizes blood into four main types: A, B, AB, and O. This classification depends on the presence of A antigens, B antigens, or both, on the surface of red blood cells. Type A blood contains A antigens, while Type B blood has B antigens. Individuals with Type AB blood possess both A and B antigens on their red blood cells. Conversely, Type O blood lacks both A and B antigens.
Plasma, the liquid component of blood, typically contains antibodies that react against antigens not present on one’s own red blood cells. For instance, Type A blood has anti-B antibodies in its plasma, and Type B blood has anti-A antibodies. Type O blood contains both anti-A and anti-B antibodies. People with Type AB blood do not produce anti-A or anti-B antibodies, which is significant for transfusions.
Understanding the Rh Factor
Beyond the ABO system, the Rh blood group system is another important factor in determining blood type. The Rh factor refers to the presence or absence of the Rh(D) antigen on the surface of red blood cells. If the Rh(D) antigen is present, the blood is considered Rh positive (e.g., A+, B+). If this antigen is absent, the blood is Rh negative (e.g., A-, B-).
The majority of people, approximately 85% of the population, are Rh positive. The remaining 15% are Rh negative.
The Genetic Rarity of AB Positive Blood
The rarity of AB positive blood stems from the specific genetic combinations required for both the ABO and Rh systems. For the ABO system, individuals inherit one allele from each parent, with A and B alleles being co-dominant and the O allele being recessive. To have AB blood, a person must inherit an A allele from one parent and a B allele from the other. This particular pairing of alleles is less common compared to inheriting two O alleles (resulting in O blood) or combinations that lead to A or B blood types.
The O allele is the most prevalent in human populations, making O blood types common. The A and B alleles are less frequent, and the necessity of inheriting both a specific A and a specific B allele simultaneously limits the occurrence of the AB phenotype. Even though the Rh positive factor is common, the low probability of inheriting the AB combination significantly impacts the overall frequency of AB positive blood. Globally, only about 3% to 4% of the population possesses AB positive blood.
AB Positive: The Universal Recipient
Despite its rarity, AB positive blood holds a unique and beneficial status as the “universal recipient” for red blood cell transfusions. This means that individuals with AB positive blood can safely receive red blood cells from donors of any ABO blood type (A, B, AB, or O) and any Rh type (positive or negative) during emergencies.
This capability is attributed to the presence of both A and B antigens, as well as the Rh antigen, on their red blood cells. Because AB positive individuals have all common antigens, their immune system does not produce anti-A, anti-B, or anti-Rh antibodies in their plasma. The absence of these antibodies prevents the immune response that would otherwise reject transfused red blood cells from other blood types.