Human blood is classified into different groups based on the presence or absence of specific protein and carbohydrate markers, known as antigens, found on the surface of red blood cells. These antigens are molecules that can provoke an immune response if perceived as foreign by the body. The ABO system is the most significant and widely recognized classification system in transfusion medicine. Understanding the ABO blood type involves recognizing the underlying genetic code, or genotype, that determines these surface markers.
Fundamentals of ABO Inheritance
The ABO blood group is determined by a single gene located on chromosome 9, which has three alternative forms, known as alleles, that an individual can inherit. These three alleles are designated \(I^A\), \(I^B\), and \(i\), with each person inheriting two alleles, one from each biological parent. The combination of these two inherited alleles dictates the specific blood type.
The relationship between these alleles follows distinct patterns of dominance. The \(I^A\) and \(I^B\) alleles are both considered dominant over the \(i\) allele, which is recessive. This means that if an individual inherits one \(I^A\) allele and one \(i\) allele, the \(I^A\) trait will be expressed, and the \(i\) trait will be masked.
When the \(I^A\) and \(I^B\) alleles are inherited together, they exhibit codominance, meaning neither allele is completely dominant over the other. Both the A and B traits are expressed equally on the surface of the red blood cells. This interaction creates the variety in the human ABO blood group system.
The Four Observable Blood Types
The various combinations of the three alleles result in four observable blood types, also known as phenotypes: A, B, AB, and O. Each blood type is defined by the specific antigens present on the red blood cell surface and the corresponding antibodies found in the plasma. Antibodies are specialized proteins that target and neutralize foreign substances.
Individuals with Type A blood have the A antigen on their red cells and possess anti-B antibodies in their plasma. Conversely, those with Type B blood have the B antigen and carry anti-A antibodies. These antibodies react against the corresponding foreign antigens, preventing incompatible cells from surviving in the bloodstream.
A person with Type AB blood expresses both the A and B antigens on their red cells but lacks both anti-A and anti-B antibodies in their plasma. Type O blood is characterized by the absence of both A and B antigens on the red cells. However, the plasma of a Type O individual contains both anti-A and anti-B antibodies, as their body recognizes both antigens as foreign.
Identifying the Six Genetic Combinations
While there are four observable blood types (phenotypes), the underlying genetics allow for six possible combinations of the three alleles, resulting in six distinct genotypes. These genotypes represent the specific pair of alleles inherited by the individual.
For Type A blood, there are two possible genotypes: the homozygous \(I^A I^A\) (both alleles are \(I^A\)) and the heterozygous \(I^A i\) (one \(I^A\) and one recessive \(i\) allele). Similarly, Type B blood can result from two genotypes: the homozygous \(I^B I^B\) or the heterozygous \(I^B i\).
Due to codominance, Type AB blood has only one possible genotype: \(I^A I^B\), where both A and B antigens are fully expressed. Type O blood, being recessive, is only expressed when an individual inherits two copies of the recessive allele, resulting in the single genotype \(i i\).
Blood Type and Transfusion Compatibility
The ABO system, including the six genotypes and four phenotypes, is significant in the context of blood transfusions. The presence or absence of specific antigens and antibodies dictates which blood types can be safely mixed. A severe reaction, known as an acute hemolytic transfusion reaction, occurs if a recipient is given red blood cells carrying an antigen that their plasma antibodies will attack.
Type O blood is often referred to as the “universal donor” because its red cells lack A and B antigens, meaning they will not be targeted by a recipient’s anti-A or anti-B antibodies. Conversely, Type AB individuals are considered the “universal recipient” because their plasma contains neither anti-A nor anti-B antibodies. Therefore, they can safely receive red blood cells from any of the four ABO types.
Type A blood can only receive blood from Type A or Type O donors, and Type B blood can only receive blood from Type B or Type O donors. This strict matching process is necessary to prevent the recipient’s immune system from destroying the transfused blood cells.