Explain the inheritance of Rh antigen .
Explain the inheritance of Rh antigen .
Rh blood group At present, the Rh system comprises 61 antigens, (1) however new antigens continue to be discovered. The Rh antigens are located on the red cell membrane protein. The D antigen is the most immunogenic and important Rh antigen, followed by c and E. Routine Rh typing of donors and patients only test for the presence/absence of the D antigen.
The presence of RhD antigen on red cells confers Rh positivity; while people who lack RhD antigen are Rh negative.
Exposure of Rh negative individuals to even small amounts of Rh positive cells, by either transfusion or pregnancy, can result in the production of Rh antibodies which are mainly IgG type.
All Rh alloantibodies should be considered potentially capable of causing (2,3,4):
- Severe haemolytic transfusion reactions (anti-D, anti-C, anti-e and anti-c)
- Haemolytic disease of the fetus and newborn (severe cases with anti-D and anti-c and mild to moderate cases with anti-C, anti-E and anti-e).
Rh Inheritance The Rh blood group system (BGS) , with more than 40 antigens, is the most complex BGS. The most significant antigen is D, followed by C, E, c, and e antigens. For more detailed information about Rh please consult resources in Suggested Reading. For purposes of understanding basic Rh inheritance, only the Fisher-Race theory will be presented here.
Fisher-Race Theory This theory is named after the two British workers who proposed it in the 1940's. Although too simplistic to explain this complex system, the theory is useful to explain routine inheritance of D, C, E, c, and e antigens. The main tenets of the theory are as follows:
- Rh inheritance is controlled by 3 closely linked loci on each chromosome of a homologous pair
- Each locus has its own set of alleles which are Dd , Cc , and Ee . The D gene is dominant to the d gene, but Cc and Ee are co-dominant. Today we know that the d gene does not exist; when used it represents absence of the D gene.
- The 3 loci are so closely linked that crossing-over does NOT occur, and the 3 genes on one chromosome are always inherited together
The Rh blood group system is attributable to two genes, RHD and RHCE, which are located on chromosome 1.
Rh positivity or Rh negativity is distinguished by testing for the RhD antigen, the expression of which depends upon whether an RHD gene has been inherited from one or both parents.
The RHD gene is dominant so a person is considered to be RhD positive whenever this gene is present, even though the gene may have only been inherited from one parent. Conversely, a person will be RhD negative if no RHD gene is inherited.
Parental Rh type Child's Rh type Positive and Positive Positive or Negative Positive and Negative Positive or Negative Negative and Negative Negative
Note: These are the various possible Rh types that children may express according to the combination of parental Rh phenotypes
.
At present, the Rh system comprises 61 antigens, (1) however new antigens continue to be discovered. The Rh antigens are located on the red cell membrane protein. The D antigen is the most immunogenic and important Rh antigen, followed by c and E. Routine Rh typing of donors and patients only test for the presence/absence of the D antigen.
The presence of RhD antigen on red cells confers Rh positivity; while people who lack RhD antigen are Rh negative.
Exposure of Rh negative individuals to even small amounts of Rh positive cells, by either transfusion or pregnancy, can result in the production of Rh antibodies which are mainly IgG type.
All Rh alloantibodies should be considered potentially capable of causing (2,3,4):
- Severe haemolytic transfusion reactions (anti-D, anti-C, anti-e and anti-c)
- Haemolytic disease of the fetus and newborn (severe cases with anti-D and anti-c and mild to moderate cases with anti-C, anti-E and anti-e).
The Rh blood group system (BGS) , with more than 40 antigens, is the most complex BGS. The most significant antigen is D, followed by C, E, c, and e antigens. For more detailed information about Rh please consult resources in Suggested Reading. For purposes of understanding basic Rh inheritance, only the Fisher-Race theory will be presented here.
Fisher-Race TheoryThis theory is named after the two British workers who proposed it in the 1940's. Although too simplistic to explain this complex system, the theory is useful to explain routine inheritance of D, C, E, c, and e antigens. The main tenets of the theory are as follows:
- Rh inheritance is controlled by 3 closely linked loci on each chromosome of a homologous pair
- Each locus has its own set of alleles which are Dd , Cc , and Ee . The D gene is dominant to the d gene, but Cc and Ee are co-dominant. Today we know that the d gene does not exist; when used it represents absence of the D gene.
- The 3 loci are so closely linked that crossing-over does NOT occur, and the 3 genes on one chromosome are always inherited together
The Rh blood group system is attributable to two genes, RHD and RHCE, which are located on chromosome 1.
Rh positivity or Rh negativity is distinguished by testing for the RhD antigen, the expression of which depends upon whether an RHD gene has been inherited from one or both parents.
The RHD gene is dominant so a person is considered to be RhD positive whenever this gene is present, even though the gene may have only been inherited from one parent. Conversely, a person will be RhD negative if no RHD gene is inherited.
Parental Rh type Child's Rh type| Positive and Positive | Positive or Negative |
| Positive and Negative | Positive or Negative |
| Negative and Negative | Negative |
Note: These are the various possible Rh types that children may express according to the combination of parental Rh phenotypes
.
