Blood typing is a fundamental process in health, identifying specific markers on the surface of red blood cells. A person’s blood type is determined by the genes passed down from their biological parents, following predictable rules of inheritance. This genetic signature is what dictates whether a person is type A, B, AB, or O, and whether their type is positive or negative. Understanding these inherited factors is necessary to ensure safe medical procedures, such as blood transfusions and organ transplants.
The Two Components of Blood Type: ABO and Rh Factor
A complete blood type is composed of two distinct systems that classify the antigens present on the red blood cell surface. The first is the ABO system, which involves two primary antigens, A and B, that determine the main blood group. A person with type B blood possesses the B antigen on their red cells, while type A has the A antigen, type AB has both, and type O has neither.
The second component is the Rh factor, which is an independent protein also known as the D antigen. The presence or absence of this D antigen determines whether the blood type is positive or negative, respectively. Therefore, a person with B+ blood has both the B antigen from the ABO system and the D antigen from the Rh system present on the surface of their red blood cells.
How Blood Type Alleles Are Inherited
Blood type inheritance involves receiving one allele from each parent for both the ABO and the Rh systems. The ABO system is governed by three possible alleles: A, B, and O. The A and B alleles are co-dominant, meaning that if both are inherited, they are fully expressed, resulting in type AB blood.
The O allele is recessive, so it is only expressed if a person inherits an O allele from both parents, resulting in type O blood. A person with type B blood could have the genetic combination of two B alleles (BB) or one B allele and one O allele (BO). In both cases, the presence of the B allele determines the blood type expression.
The Rh factor is inherited separately from the ABO group and follows a simpler dominant-recessive pattern. The allele for Rh-positive (D) is dominant, and the allele for Rh-negative (d) is recessive. To be Rh-positive, a child only needs to inherit one dominant D allele from either parent.
A person is only Rh-negative if they inherit the recessive d allele from both parents. Since the ABO and Rh factors are inherited independently, the combination of the two systems results in the eight common blood types. The genetic rules for both systems must be fulfilled simultaneously to determine the final blood type.
Parental Pairings That Result in B+ Blood
For a child to have B+ blood, two distinct genetic requirements must be met: the child must inherit at least one B allele and at least one Rh-positive (D) allele. The B allele can be inherited from a parent who is genetically BB, BO, or AB. For example, a parent with type O blood (OO) and a parent with type AB blood (AB) could produce a child with type B blood by inheriting the B allele from one parent and the O allele from the other.
A child can receive the B antigen from parents who are B/B, A/B, or O/B, provided at least one B allele is passed down. The only parental combinations that cannot result in a type B child are those where neither parent possesses the B allele. This includes two parents who are both type A, two parents who are both type O, or a type A and a type O parent.
The positive aspect of the B+ blood type requires the child to receive the dominant Rh-positive allele from at least one parent. If both parents are Rh-positive, the child has a high probability of being B+, though they could still be B- if both parents carry and pass on the recessive Rh-negative gene. If one parent is Rh-positive and the other is Rh-negative, they can still produce a B+ child as long as the Rh-positive parent contributes the dominant D allele. A B+ child is genetically impossible only if both parents are Rh-negative, as they would have no dominant D allele to pass on.
Transfusion Compatibility for B+ Individuals
The antigens present on B+ red blood cells dictate which blood types can be safely received during a transfusion. A B+ individual possesses the B antigen and the Rh D antigen, but their plasma contains antibodies against the A antigen. Therefore, they can only safely receive blood that lacks the A antigen.
A B+ recipient can receive blood types that are compatible because they lack the A antigen:
- B+ blood
- B- blood
- O+ blood
- O- blood
A B+ individual can donate red blood cells to others who are B+ or AB+, as these types already possess the B and Rh D antigens, preventing an adverse reaction.