Human blood is categorized into different types based on the ABO blood group system, a classification discovered by Karl Landsteiner in 1901. This system defines four primary blood types: A, B, AB, and O. These classifications are important for medical procedures such as transfusions. Blood types are determined by specific biological markers and are passed down through generations.
Genetic Inheritance of Blood Types
Blood types are inherited from parents. The ABO blood group system is controlled by a single gene located on chromosome 9, which has three main forms, known as alleles: A, B, and O. The A allele leads to the production of A antigens, and the B allele leads to the production of B antigens. The O allele, however, does not produce either A or B antigens.
The relationship between these alleles determines a person’s blood type. Both the A and B alleles are dominant over the O allele; for example, inheriting an A allele and an O allele results in blood type A. Similarly, inheriting a B allele and an O allele results in blood type B. A and B alleles are codominant; if both are inherited, both A and B antigens are expressed, resulting in blood type AB. This explains how parents with certain blood types can have children with different, yet predictable, blood types, such as two heterozygous parents with type A and type B blood potentially having a child with type O blood.
Blood Type Markers
The classification of ABO blood types relies on the presence or absence of specific sugar molecules called antigens on the surface of red blood cells. These antigens act as identifiers for the body’s immune system. Antibodies are proteins found in the liquid part of blood, called plasma, which recognize and target foreign antigens.
Individuals with blood type A have A antigens on their red blood cells and anti-B antibodies in their plasma. Conversely, those with blood type B possess B antigens on their red blood cells and anti-A antibodies in their plasma. People with blood type AB have both A and B antigens on their red blood cells but do not have anti-A or anti-B antibodies in their plasma. Blood type O is distinct because individuals with this type have neither A nor B antigens on their red blood cells, but they have both anti-A and anti-B antibodies in their plasma.
Blood Transfusion Compatibility
The presence or absence of specific antigens and antibodies dictates the compatibility of blood for transfusions. When incompatible blood types are mixed, the recipient’s antibodies can recognize the donor’s red blood cell antigens as foreign, triggering an immune response that can cause the red blood cells to clump together. This reaction can be life-threatening.
Blood type O negative is often referred to as the “universal donor” because its red blood cells lack A, B, and Rh antigens. This makes O negative blood safe to transfuse to individuals of any blood type. In contrast, blood type AB positive is considered the “universal recipient” because individuals with this blood type have both A and B antigens on their red blood cells and no anti-A or anti-B antibodies in their plasma.