ABO Rh Typing

ABO System

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 (O_h)
    • Produces anti-H antibodies
    • Appears as Group O on T&S, but antibody screens and panels will be fully positive

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

	hh	H	A	B
Gene Product (Enzyme)	None	L-fucosyltransferase	N-acetylgalactosaminyltransferase	D-galactosyltransferase
Dominant/Terminal Sugar	None (only precursor)	L-fucose	N-acetyl-galactosamine	D-galactose
Groups	Oh (Bombay)	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	IgM
	Anti-B	IgM
	Anti-A,B	IgG

IgG antibodies can cross placenta, but IgM usually doesn't (except in trauma)

Secretory Status

• A, B, and H antigens can be soluble
• Se gene confers secretory status
  • Dominant gene (FUT-2)
  • Fucose added to type 1 precursor
    • Type 1 precursor found in body fluids and secretions
    • Type 2 precursor found on RBCs, body fluids, and secretions
  • H antigen is then secreted, as are any A and B antigens attached to it

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 auto-agglutination 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)

Rh System

• Second most immunogenic after ABO system
• RHD gene and RHCE gene
  • Code directly for antigens
• Red cell immune, well-developed at birth
  • No antibodies produced unless exposed to RBCs
  • IgG sensitization
  • Anti-D can develop due to pregnancy or transfusion
• Rh null is rare - no Rh antigens produced
  • Caused by membrane abnormalities or processing defects

Weiner	Fisher-Race	Weiner	Fisher-Race
R0	Dce	r	dce
R1	DCe	r'	dCe
R2	DcE	r''	dcE
Rz	DCE	ry	dCE

Testing

• IAT testing performed to detect IgG
  • Recommended to use two different reagents
    • Monoclonal IgM Anti-D
    • Polyclonal IgG Anti-D
  • Rh Control containing only diluent (e.g., bovine albumin) without anti-D
    • Done to ensure there is no false agglutination (e.g., cold antibodies, etc.)

Weak D Testing

• Most to least D antigen: -D- > R²R² > R¹R¹ > R¹r or R⁰r > R¹r' or R⁰r'
• Causes:

1. Genetic weak D: gene codes for fewer D antigen → IAT weak-D testing
2. C-trans (positional) weak D: C antigen is in trans position to D antigen, blocking production of D causing weaker expression → use monoclonal anti-D
3. Partial D/Mosaic D: mutation causes part of Rh gene to be missing → detect by monoclonal anti-D, may need IAT weak-D testing (results may different with different antisera)
   • Individuals can produce Anti-D to the part of the antigen that is missing - require Rh Negative blood!

• Weak D testing not usually performed for recipients (assume patient is Rh Negative), but must be performed for donor testing
  • Weak D testing performed on all Rh Negative babies from Rh Negative moms to ensure baby is actually Rh Negative (if Weak D, mother could produce Anti-D)

Weak D Molecular Testing

• Performed for patients with special blood needs (certain prenatal patients, chronic transfusions where genotyping may modify blood products used)
• Can determine whether patient has variants that can cause Anti-D production

Rh and Transfusion

• Rh Negative individuals may produce Anti-D when exposed to Rh Positive blood, but it is not guaranteed
  • Can receive Rh Positive cells at least once
  • Anti-D is IgG and doesn't bind complement
    • Extravascular hemolysis would occur
• Other Rh antigens aren't of concern unless the patient has an antibody to one or more of those antigens
• Patients receiving many transfusions require special consideration (e.g., sickle cell, thalassemia, etc.)