Rh Blood Group System: Part II

Renee Newman Wilkins, Ph.D., MLS(ASCP)CM

CLS 325/435School of Health Related ProfessionsUniversity of Mississippi Medical Center

Rh Antigens:

D, C, E, c, and e

D antigen

The D antigen is a potent immunogen (elicits an immune response)…why?Structure and number of sitesSome genotypes have more D antigens

than others(D--/D--) has more antigens than a

genotype with only one D gene (especially when C is on the opposite chromosome)

Rh Typing

Routine Rh typing for donors and

patients involves typing for only the

D antigen

D Testing

Routine D antigen testing involves testing the patient RBCs with anti-D commercial antisera

If the D antigen is present, it should agglutinate strongly with anti-D at Immediate Spin (IS)If you’re Rh+, you have the D antigenIf you’re Rh-, you do not have the D

antigen

Weak D phenotype

Some D-positive RBCs DO NOT react at Immediate Spin using commercial anti-D

In these cases, AHG testing is needed to determine the D status

Weak D testing

If negative at IS, patient cells and anti-D reagent are incubated at 37° for 20 minutes, then centrifuge

If still negative, wash x3 and add AHG If negative, add CC and report as Rh

negative (if CC agglutinate) If positive, report as Weak D Positive

Patients who require AHG testing to determine the presence of the D

antigen are called “Weak D Positive”

RBC with normal amounts

of D antigen

Weak D (Du)

Weak D (Du) Phenotype

Weak D can be inherited in three ways:Incomplete/Partial antigen (D mosaic)Due to the position effectWeakened expression of D

Partial D (D Mosaic)

Missing one or more parts of the D antigen

Since the antisera is specific for the whole D antigen, a weak reaction may result if patient has a partial antigen

Why is D mosaic significant?

D Mosaic/Partial D

If the patient is transfused with D positive red cells, they may develop an anti-D alloantibody* to the part of the antigen (epitope) that is missing

*alloantibody- antibody produced with specificity other than self

Missing portion

RBC RBC

Position Effect

Gene interaction effect C allele is in trans position to D allele Does not occur when C is in cis position Steric hindrance causes the anti-D

reagent to weakly attach (C antigen crowds the D antigen)

D positive blood can be given to patient

Position Effect

C in trans position to D:

D c e / d C e

C in cis position to D:

D C e / d c e

Weak D

NO weak D

Weakened D (Genetic)

Inherited weak D More common in African Americans These individuals may inherit the RHD

gene that encodes for a weaker expression (Dce haplotype)Quantitative, meaning less is

expressedDetected best at AHG phase

Unusual Phenotypes

D-DeletionRh null

D-Deletion

No reaction when RBCs are tested with anti-E, anti-e, anti-C or anti-c

Requires transfusion of other D-deletion red cells, because these individuals may produce antibodies with single or separate specificities

Written as D- - or -D-

Rh Null

This condition occurs when the red cells have no Rh antigen sites

The lack of antigens causes the red cell membrane to appear abnormal leading to: Stomatocytosis Hemolytic anemia

2 Rh null phenotypes: Regulator type – gene inherited, but not

expressed Amorph type – RHD gene is absent, no

expression of RHCE gene

AABB Standards

Require weak D testing on all donor red blood cells that do not agglutinate at IS

DO NOT require weak D testing on recipient blood each facility has their own protocol If only IS is performed and patient is

negative, they will receive negative units However, some labs don’t like to waste D-

negative units, so they take the test to AHG If the patient is positive, they may receive D-

positive units (it would be rare that the patient is a Partial D)

Other Rh antigens

Compound antigens are cis-product antigens (f, rh1, etc)f antigens occur when c and e are

found in cis (Example: dce/dce)rh1 or Ce antigens occur when C and

e are in cis (example: G antigens

Genes that code for C or D also code for G; anti-G mimics anti-C and anti-D

Other Rh antigens

Cw antigen low frequency antigen found in only 2% of

Whites and rare in Blacks Most individuals who are C+ are Cw+ Antibodies to these antigens can be naturally

occuring and may play a role in HDN and HTR

Others: more antigens can be found in Table 5-7 of your textbook (page 115)

Rh Antibodies

Not naturally occurring Produced as a result of immune

stimulation from exposure to the antigen through transfusion and/or pregnancyImplicated in transfusion reactionsBecause they are IgG, they can cross

placenta and cause HDN

Antibody Characteristics

Generally IgG (except anti-E) React stronger (show dosage) when antigens

are homozygous (C+c-) vs heterozygous (C+c+) Warm reacting

Best at AHG Occasionally at 37° phase Rarely at IS (room temp)

Clinically significant Because they react at body temp.