The human body uses blood type as a fundamental identifier, referring to the presence or absence of specific surface markers, known as antigens, on red blood cells. The two most recognized systems are the ABO and Rh factors, which determine compatibility for medical procedures like blood transfusions. For the vast majority of people, this biological characteristic remains fixed throughout life, serving as a permanent genetic signature. However, one significant medical intervention provides a lasting exception to this rule, alongside several temporary scenarios that can confuse blood typing tests.
How Blood Type is Determined at Birth
Blood type is a trait inherited from both parents, determined by genes located on specific chromosomes. The ABO blood group system is governed by the ABO gene on chromosome 9, which has three versions, or alleles: A, B, and O. The A and B alleles are co-dominant, meaning both are expressed if inherited, while the O allele is recessive.
The presence of the A or B allele causes the production of enzymes that add specific sugar molecules (the A and B antigens) to the surface of red blood cells. If a person inherits the O allele from both parents, their red blood cells have neither the A nor B antigen.
The Rh factor, which determines if a blood type is positive or negative, is governed by a separate set of genes, primarily on chromosome 1. If you inherit at least one dominant Rh-positive allele, you are Rh-positive; you must inherit two recessive Rh-negative alleles to be Rh-negative. This genetic blueprint establishes the fixed blood type that the body’s stem cells continually produce throughout life.
Medical Situations That Cause Temporary Shifts
While the genetic code for blood type is fixed, certain medical events can temporarily alter the appearance of a person’s blood type in laboratory tests. A massive blood transfusion can lead to a phenomenon called a “mixed field” reaction.
This occurs when the recipient’s circulatory system contains a significant population of both their original red blood cells and the transfused donor cells. For example, a person with type A blood receiving type O blood may temporarily type as O because the donor cells are numerous. As the transfused blood cells die off, the patient’s bone marrow continues to produce cells matching their original genetic type, and the typing results return to normal within a few months.
Another unusual, non-permanent change is the “acquired B phenomenon,” which can happen in patients with severe bacterial infections, such as those associated with colon cancer or sepsis. In this situation, an enzyme produced by the bacteria chemically alters the A antigen on the red blood cell surface, making it closely resemble the B antigen. This temporary modification can cause the blood to incorrectly type as AB or B until the underlying infection is successfully treated.
Permanent Blood Type Change Through Stem Cell Transplants
The one definitive way a person’s blood type can be permanently altered is through an allogeneic Hematopoietic Stem Cell Transplant (HSCT), commonly known as a bone marrow transplant. This procedure is used primarily to treat certain cancers, such as leukemia and lymphoma, and other blood disorders.
The process involves first destroying the patient’s existing, diseased blood-producing cells in the bone marrow using chemotherapy and radiation. The patient is then infused with healthy hematopoietic stem cells harvested from a donor. These donor stem cells engraft in the recipient’s bone marrow and begin hematopoiesis, the formation of new blood cells.
Since red blood cells are produced exclusively by these stem cells, they will now carry the antigens corresponding to the donor’s blood type. If the donor and recipient have different blood types, the recipient’s blood type will eventually convert to that of the donor. This conversion is complete when all of the patient’s original red blood cells have been naturally replaced by the new, donor-derived cells, a process that takes several months. While the patient’s non-blood tissues maintain their original genetic blood type, the circulating red blood cells now permanently match the donor’s type.
Scientific Research into Blood Type Conversion
Beyond clinical transplants, scientists are researching experimental methods to change blood type, primarily to increase the universal blood supply. The focus of this research is on using specialized enzymes to chemically remove the A and B antigens from red blood cells in a laboratory setting.
Converting A or B blood to type O is a goal because type O red blood cells are considered the universal donor, safely transfusable to almost any patient. Researchers have identified enzyme cocktails, often sourced from gut bacteria, that are effective at stripping the sugar structures defining the A and B antigens. These enzymes convert the A or B blood into “enzyme-converted to O” or “ECO” blood.
Recent advances have developed enzymes that are up to 30 times more efficient than earlier attempts, successfully converting entire units of blood in a short amount of time at room temperature. These methods are currently limited to treating donated blood outside of the body and are not yet viable for routine clinical application within a living patient. Ongoing research aims to ensure that the modified red blood cells remain safe and functional for transfusion.