ABO Rh Typing
The ABO system is the most clinically significant blood group system in transfusion medicine.
- Carbohydrate antigens
- Non-RBC stimulated: individuals possess ABO antibodies to the antigens that they lack
- IgM
- Cause severe transfusion reactions
- Cell lysis and hemolysis
ABO Genes
- A and B genes present on chromosome 9
- Code for transferase enzymes, which transfer sugar to a precursor on the RBC membrane
- A and B co-dominant, O is recessive
- One allele received from each parent
- Cis-AB inheritance
- Rare situation where A and B genes end up on same chromosome during crossover
- Can result in AB mother and OO father having AB baby
H Gene
- Present on chromosome 19
- H gene codes for a fucosyl transferase (FUT 1) that produces H antigen
- Precursor to A and B antigens
- H allele frequency >99.99%
- hh is Bombay phenotype
- Produces anti-H antibodies
- hh is Bombay phenotype
ABO Antigens
- Carbohydrates attached to RBCs via gene-encoded transferase
- A and B antigens are sugars attached to terminal end of oligosaccharides
- O group has H terminal antigen instead of A or B
| H | A | B | |
|---|---|---|---|
| Gene Product (Enzyme) | L-fucosyltransferase | N-acetylgalactosaminyltransferase | D-galactosyltransferase |
| Dominant/Terminal Sugar | L-fucose | N-acetyl-galactosamine | D-galactose |
| Groups | A, B, AB, O | A, AB | B, AB |
Most to least H-antigen: O > A2 > B > A2B > A1 > A1B (subgroups used for organ donation)
- Most people are A1
ABO Antibodies
- Develop 'naturally' regardless of exposure to blood products (may be similar to structures found in bacteria, pollen, etc.)
- Non-red cell immunity
- Takes time to develop (3-6 months) in infants
| Blood Group | Antibodies Present | Class |
|---|---|---|
| Group A | Anti-B | IgM |
| Group B | Anti-A | IgM |
| Group AB | None | |
| Group O | Anti-A
Anti-B Anti-A,B |
IgM
IgM IgG |
IgG antibodies can cross placenta, but IgM usually doesn't (except in trauma) ABO Typing
- Babies: perform forward testing ONLY
- Babies: if baby appears Rh Negative and the mother is Rh Negative, perform weak D testing
- AB Rh Positive patients require Rh Control to be performed
- RHC should be a reaction of 0. This checks that the agglutination seen is not a result of autoagglutination in the patient.
- Rh Control is also used for discrepancies (Rh testing results don't match, e.g. Anti-D1 is 2+ and Anti-D2 is 0)
- Rh Control is also used for IAT Testing (e.g., Weak D testing)