Blood is categorized into different types based on specific markers present on red blood cells. These classifications are crucial for medical procedures, particularly blood transfusions, where compatibility between donor and recipient is paramount to prevent adverse immune reactions.
Prevalence of AB Negative Blood
AB negative is the rarest of the eight primary blood types. In the United States, less than one percent of the population has AB negative blood, with some estimates placing it at approximately 0.6%. Globally, AB negative consistently remains very rare.
Understanding AB Negative Blood
Blood types are determined by the presence or absence of specific antigens on the surface of red blood cells, as classified by the ABO and Rh blood group systems. In the ABO system, AB blood means that both A and B antigens are present on the red blood cells. Unlike other blood types, AB blood also lacks both A and B antibodies in the plasma. The “negative” in AB negative refers to the Rh factor, indicating the absence of the RhD antigen. Therefore, a person with AB negative blood possesses both A and B antigens but does not have the RhD antigen.
Significance in Transfusions
The distinct characteristics of AB negative blood have important implications for medical transfusions. Individuals with AB negative blood can only receive red blood cells from donors who are also Rh-negative, specifically from AB negative, A negative, B negative, or O negative types. Receiving Rh-positive blood would trigger an immune response due to the recipient’s lack of the RhD antigen.
Conversely, AB negative plasma is highly valuable in medical settings. It is considered a “universal donor” plasma, meaning it can be transfused to patients of any blood type. This is because AB negative plasma does not contain A or B antibodies, which would otherwise react with the recipient’s red blood cells. This makes AB negative plasma particularly useful in emergency situations when a patient’s blood type is unknown.
Genetic Basis of Rarity
The rarity of AB negative blood stems from its specific genetic inheritance patterns. Blood types are inherited from parents, with each person receiving one allele (gene variant) for the ABO system and one for the Rh factor from each parent. For an individual to have AB blood, they must inherit an A allele from one parent and a B allele from the other. The A and B alleles are co-dominant, meaning both are expressed.
Being Rh negative requires inheriting a recessive Rh-negative allele from both parents. Since the alleles for A and B antigens are less common in many populations compared to the O allele, and the Rh-negative allele is also less prevalent than the Rh-positive allele, the combined probability of inheriting both the AB combination and two Rh-negative alleles makes AB negative blood inherently rare. Globally, only about seven percent of people are Rh negative.