Blood type is a classification system based on inherited markers on the surface of red blood cells, categorized primarily by the ABO system (A, B, AB, or O) and the Rh system (positive or negative). For most people, blood type is fixed from conception and cannot change. However, modern medicine and certain severe health conditions introduce extremely rare circumstances where the blood type can shift, either permanently or transiently.
The Genetic Basis of Permanent Blood Type
A person’s blood type is determined by the genetic code (genotype) inherited from their parents, which dictates the specific antigens produced on the surface of red blood cells. The ABO system involves three possible alleles—A, B, and O—with everyone inheriting one from each parent. The combination of these two alleles determines the resulting blood group.
The A and B alleles instruct the body to create the A and B antigens, respectively, while the O allele results in no antigen production. Because this genetic code is stable, the blood type is considered a permanent, lifelong characteristic. This foundation ensures the body consistently produces red blood cells with the same surface markers.
Understanding the Rh Factor: Positive vs. Negative
The second major classification, the Rh system, is responsible for the positive or negative designation. This designation is based solely on the presence or absence of the D antigen (RhD antigen) on the red blood cell surface. If the D antigen is present, the blood is Rh positive (Rh+); if it is absent, the blood is Rh negative (Rh-).
The instruction to produce the D antigen is inherited genetically. Since the Rh factor is determined by a fixed genetic code, it typically remains constant throughout life. Combining this factor with the ABO system creates the eight common blood types.
Medical Exceptions: When Blood Type Appearance Can Shift
While the genetic code is fixed, rare medical interventions and conditions can alter the resulting blood type. The most significant cause of a permanent change in both ABO and Rh type is a bone marrow or hematopoietic stem cell transplant. This procedure replaces the recipient’s blood-producing cells with a donor’s cells.
If a patient with A-positive blood receives stem cells from an O-negative donor, the patient will eventually produce O-negative red blood cells. This change occurs gradually as old red blood cells die off and are replaced by new cells derived from the donor’s marrow. The complete transition, where the blood type fully matches the donor’s, can take several months to a year.
Other scenarios involve transient alterations that affect how the blood type is tested, rather than a true genetic change. A massive blood transfusion can temporarily cause the patient’s blood to test as the donor’s type. This effect is short-lived, as the patient’s own bone marrow quickly replaces the transfused cells with their original type.
Rarely, a severe bacterial infection can lead to the “acquired B antigen” phenomenon. Certain bacteria produce an enzyme that chemically modifies the A antigen on type A red blood cells, making it resemble the B antigen during testing. This temporary misclassification resolves once the underlying infection is successfully treated.
Genotype Stability Versus Phenotype Expression
The distinction between a person’s genotype and phenotype helps explain these rare exceptions. The genotype refers to the underlying, inherited genetic code for the blood type, present in almost every cell. The phenotype refers to the physical expression of that code, specifically the antigens displayed on the surface of the red blood cells.
Under normal circumstances, the genotype and phenotype are matched and stable. A bone marrow transplant permanently changes the genotype of the blood-forming tissue to the donor’s type. Conversely, conditions like the acquired B antigen only alter the phenotype (the physical appearance of the antigen) without changing the patient’s core genetic instructions. The normal phenotype returns once the external factor is removed.