View source for 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

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

{| class="wikitable"
|+
!
!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

{| class="wikitable"
|+
!Blood Group
!Antibodies Present
!Class
|-
|Group A
|Anti-B
|IgM
|-
|Group B
|Anti-A
|IgM
|-
|Group AB
| colspan="2" |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)