Rh Blood Group System

What is required for the production of anti-D and other Rh antibodies?

• immune stimulation

  • via positive red cells during pregnancy or after transfusion

  • therefore NOT naturally occurring

What is the current theory for Rh blood group system genetics?

two closely linked genes (codominant alleles) control the expression of all Rh antigens

What is the RHD gene responsible for?

• determining the expression of D antigen

  • D antigen is very immunogenic (if an Rh negative individual receives Rh positive red cells, anti-D antibody will be created)

What is the RHCE gene responsible for?

determining the expression of the C, c, E, and e antigens

What does the Fisher-Race theory of Rh system genetics state?

• Rh antigens are controlled by 3 closely linked genes

  • inherited as a haplotype

  • one gene for D/d, one gene for C/c, and one gene for E/e

What does the Wiener theory of Rh system genetics state?

• Rh antigens are controlled by alleles at one gene locus

  • basically, one gene expresses all Rh antigens

  • 8 alleles exist at a single gene locus

The genotype of an individual can be predicted by knowing:

• the race of an individual

  • common genotypes of black and white individuals: R1, R0, R2, and r

  • rarely encountered: r’, r’’, Rz, and ry

Are Rh antibodies clinically significant?

• YES

  • D-negative individuals should never receive D-positive blood because anti-D can be produced

weak D antigens

• significantly decreased quantities of the D antigen on red cells

• requires an IAT for detection

  • should be performed after the forward type shows a reaction of 2+ or less

  • IAT reaction should be 3+ or 4+

Weak D testing can only be performed on those with a ___ DAT.

• negative (no IgG on red cells in vivo)

  • positive DATs can cause false positives

What are 3 possible causes of weak D antigens on red cells?

• genetics

• the position (Ceppellini) effect

• the presence of partial D antigens

How can genetics cause weak D antigens?

• can occur due to the inheritance of a weaker form of the D antigen (reduced D antigen sites on RBCs)

• these individuals can receive D-positive cells without making anti-D

• more common in the black population

How can the position effect cause weak D antigens?

• weak D can occur if D and Ce antigens are inherited on different chromosomes (D and Ce in trans position)

• D antigen may appear weak because the D antigen is inherited trans to D

• individuals can receive D positive cells without making anti-D

How can the inheritance of partial D antigens cause weak D antigens?

• partial D antigens are D antigens (individuals are D positive) that have missing parts

• when exposed to whole D antigens, they may produce anti-D toward the missing part

• partial D antigens react strongly with monoclonal reagents, just like complete D antigens

When should partial D antigen be suspected?

• when a D positive individual makes anti-D that does NOT react with their own red cells

• when two different manufacturers’ anti-D reagents do not yield results that agree

The AABB requires that ___ donor cells be tested for weak D antigen even if they are initially ___ in forward typing.

all, nonreactive

Why is a control required for weak D testing?

• because it is an indirect antiglobulin test (IAT)

  • patient cells are incubated with anti-D, washed, and then AHG reagent is added

  • weak D testing is NOT required for patients with a positive DAT (false-positives can result)

What are compound antigens?

• antigens that form when certain genes code for an additional protein

• the f antigen is compound

  • this antigen is only present if c and e antigens are inherited together as a haplotype (R0 or r)

  • anti-f formation is possible and is found among R1R2 individuals

G antigen

• present on red cells with C and D antigen

  • those who lack C and D can create anti-G antibody

  • almost all genes that code for C or D antigen will code for G antigen

D-deletion (D-)

• unusual Rh phenotype

• D antigen is the only Rh antigen present (all others are deleted) on red cells

• during forward typing (antisera testing), anti-D will be positive but:

  • anti-E, anti-e, anti-C, and anti-c are negative

Rh null

• unusual Rh phenotype

• red cells have NO Rh antigens

• cells have membrane abnormalities that shorten their survival, causing hemolytic anemia

What are the two possible genetic causes of the Rh null phenotype?

• absence of the regulator gene RHAG

  • gene must be present for Rh antigens to form

• individual is amorphous for all Rh antigens

Rh mod

• unusual Rh phenotype

• most Rh antigen expression is missing

• may occur due to the inheritance of a modified RHAG (regulator) gene

• associated with hemolytic anemia

What class of antibody are most Rh antibodies?

• IgG

  • antigen-antibody reactions occur at 37 C

  • agglutination is best observed at the IAT phase/test

True or false: using potentiators like LISS or PEG are beneficial for identifying Rh antibodies

TRUE

C, c, E, and e antibodies react stronger to red cells with ___ antigen expression.

homozygous

• Rh antibodies display dosage

Why is E negative AND c negative blood given when only anti-E is identified in a patient?

because weak anti-c antibody is often seen together with anti-E

Why do most Rh antigens NOT activate complement?

• because Rh antigens are too far apart on red cells for IgG to attach and activate complement

• RBC destruction due to Rh antibody is primarily extravascular

Remember: Rh antibody levels can become undetectable over time, but can produce a rapid response upon exposure to the antigen

• this is why it’s important to check the patient’s history for previously identified Rh antibodies

• RBCs negative for the corresponding antigen are required for transfusions

What is HDFN and how can it happen?

• a hemolytic disease that can occur when a woman is Rh negative and the fetus/infant is Rh positive

• how it happens:

  • Rh-negative mother produces anti-D during her first pregnancy if fetal and maternal blood mix before or during delivery (first pregnancy is safe)

  • subsequent pregnancies with Rh-positive fetuses cause maternal IgG (anti-D) to cross the placenta, attach to red cells, and destroy them

What can be given to Rh negative mothers after delivery to prevent HDFN?

• RhIG or Rhogam

  • protects D-negative mothers from producing anti-D

What is the difference between weak D antigen and weakly reacting anti-D?

• weak D antigen

  • refers to the antigen that shows weak reactions with anti-D, IAT is needed for detection

• weakly reacting anti-D

  • antibody that reacts weakly to D antigen (seen in women taking Rhogam)

A patient has a history of an alloanti-f. Because commercial antiserum is unavailable, what is the best course of action for locating compatible RBC units?

• crossmatch c-negative units

  • f antigen is inherited alongside c and e antigen.

  • Since the patient does not have f antigen, they also do not have c antigen and should receive c negative units.

Which of the following units would react negatively with anti-G?

R0r

rr

R1r

r’r

• rr

Which of the following genotypes is heterozygous for the C antigen?

R1r

R2R2

R1R1

r’r’

R1r

agglutinogen

a group of antigens agglutinated by antisera

Rouleaux is a false-positive reaction caused by elevated serum serum protein levels. What two tests could be affected by an elevated protein level?

• ABO reverse typing

• ABO forward typing (if unwashed cells are being used)

What is the purpose of having an Rh control in routine ABO/Rh typing?

• to determine the validity of IS positive reactions for AB patients

  • patients whose typing results show A positive, B positive, and D positive