Rh Blood Group System, Phenotyping, and Weak D Testing

Flashcards covering Rh phenotyping controls, Tippett's theory, population frequencies, RhoGAM protocol, and weak D testing mechanisms from the lecture.

What type of red blood cell must be selected as a positive control when performing Rh antigen phenotyping?

A heterozygous cell (such as big E, little e) must be used rather than a homozygous cell to verify reagent sensitivity.

What are the four clinical reasons for performing red blood cell phenotyping mentioned in the lecture?

Determining parentage, predicting hemolytic disease of the fetus and newborn (HDFN), confirming Rh specificity, and finding compatible blood for transfusion.

What are the three specific categories of hemolytic disease of the fetus and newborn (HDFN)?

ABO, Rh, and IgG.

What method is currently considered the gold standard for blood bank antigen testing?

Testing with antisera reagents.

According to Tippett's theory proposed in 1986, which two closely linked genes determine Rh expression?

The $RHD$ gene (which produces the D antigen) and the $RHCE$ gene (which produces the C/c and E/e antigens).

What is the most common Rh phenotype/genotype in the Caucasian population?

Big D, big C, little e / little d, little c, little e ($R_1r$), which accounts for approximately $31\%$ of Caucasians.

What is the most common Rh phenotype/genotype in the Black population?

$R_0r$ (big D, little c, little e / little d, little c, little e), accounting for approximately $46\%$ of the population.

How does the cellular location of the Rh D antigen differ from most other red blood cell antigens?

While most antigens reside exclusively on the outer membrane surface, the D antigen is located both on the red cell surface and encrypted deeper within the red cell membrane.

Can Rh antibodies occur naturally in plasma?

No, Rh antibodies are never naturally occurring; they are immune antibodies that only develop through exposure from pregnancy or transfusion.

What are the approximate population frequencies of the big E and little e antigens?

Approximately $30\%$ of individuals carry the big E antigen, whereas almost all ($98\%$) carry the little e antigen.

Why are sickle cell disease patients routinely phenotyped for Rh antigens?

Because sickle cell disease patients require chronic transfusions, making them susceptible to forming Rh antibodies such as anti-C or anti-e.

At what gestational age is antenatal RhoGAM routinely administered to an Rh-negative mother?

At $28\text{ weeks}$ gestation, because RhoGAM has a circulating lifespan of $12\text{ weeks}$ and full term is $40\text{ weeks}$.

Within what timeframe postpartum must RhoGAM be administered if an Rh-negative mother delivers an Rh-positive or weak D baby?

Within $72\text{ hours}$ (or $3\text{ days}$) of delivery.

What testing must be verified before administering RhoGAM at 28 weeks gestation and at 72 hours post-delivery?

An antibody screen must be performed to confirm that the mother has not already developed an active immune anti-D antibody.

How does RhoGAM administration protect future pregnancies without causing fetal anemia?

It supplies passive anti-D that tricks maternal immunity into sensing protection, allowing sensitized fetal cells to be cleared by the reticuloendothelial system before maternal immune anti-D is formed.

Which Canadian province implemented non-invasive prenatal molecular testing on maternal blood to assess fetal Rh status?

Ontario.

Which two patient populations automatically reflex to weak D testing if their initial Rh typing is negative?

First-time blood donors and Rh-negative babies born to Rh-negative mothers.

What are the three mechanisms that cause a weak D phenotype?

Genetic weak D, positional effect (gene interaction / trans effect), and partial D (mosaic D).

What historic terminology was formerly used to denote weak D?

$D^u$.

What laboratory steps are required during weak D testing following immediate spin?

Incubation at $37^\circ\text{C}$, centrifugation and reading, washing $4\text{ times}$, addition of antihuman globulin (AHG / Coombs serum), and confirmation of negative results with Coombs control check cells.

How must an individual who tests weak D positive be interpreted and reported?

They are always interpreted and reported as Rh positive, never as weak D positive.

How does the trans positional effect weaken D antigen expression?

When the big C antigen is inherited on the opposite chromosome from the D antigen (in trans position, such as $R_0/r'$), it sterically interferes with D antigen expression, occurring in less than $2\%$ of Caucasians.

What is partial D (mosaic D)?

An inherited anomaly where an individual lacks one or more epitopes of the D antigen complex, enabling them to form an anti-D-like antibody against the missing piece if exposed to complete D red cells.

What two clinical situations should be suspected if an Rh-positive person presents with a positive anti-D antibody screen?

Either the patient received passive RhoGAM recently (e.g., during pregnancy), or the patient has a partial D (mosaic D) phenotype and produced an antibody against the missing D epitope.

What is the relative hierarchy of D antigen density on red blood cells from highest concentration to lowest?

$R_2R_2$ has the highest density of D antigen sites, followed by $R_1R_1$, down to $R_0r$.

How is an individual with a weak D phenotype classified when acting as a blood donor versus when needing a transfusion as a recipient?

As a blood donor they are classified and labeled as an Rh-positive donor, but as a hospital transfusion recipient they receive Rh-negative blood.