ABO Group and Rh Type

Rh type is needed during pregnancy to assess the risk of hemolytic disease of the fetus and newborn (HDFN). In this condition, fetal red blood cells are destroyed by antibodies that are formed in the pregnant woman and that are able to cross the placenta into the fetal blood.

The most common type of severe HDFN is due to Rh group incompatibility, in which the pregnant woman lacks the D antigen of the Rhesus system (Rh negative or D-), while her fetus inherits the D antigen from the father and carries it on the red blood cells (Rh positive or D+). Other Rh antigens of concern during pregnancy are C and E antigens. Each has a big-letter and a small-letter allele (C,c and E,e). Incompatibility of these alleles between the pregnant woman and fetus can also cause HDFN in rare instances, with anti-c being associated with cases of severe HDFN. However, the pregnant woman’s cells are not routinely tested for presence of these Rh antigens. If a maternal antibody screen reveals the presence of an antibody that may be directed to C, c, E or e antigens, then documenting the absence of such antigens from the maternal cells would aid in the characterization of this antibody.

ABO incompatibility between the pregnant woman and fetus is the most common cause of HDFN today. In this condition, the pregnant woman’s blood type is group O. People with O blood group have naturally occurring antibodies to A and B blood group antigens. Much of the time, these are of the IgM antibody class and are too large to cross the placental barrier into the fetal blood. Rarely, smaller IgG class antibodies to A or B antigens can form in the pregnant woman and can cross the placental barrier, causing HDFN when the fetus has inherited either A or B blood group antigens from the father. HDFN is most commonly seen in those infants of group O mothers because these mothers have the highest titers of IgG anti-A, anti-B, and anti-A,B antibodies. Nevertheless, this form of HDFN is usually milder than that caused by Rh incompatibility.

There are 3 possible mechanisms through which the pregnant woman can develop antibodies to fetal red blood cells:

1. Fetal-maternal hemorrhage, during which the mother is exposed to fetal red cells. This can occur due to miscarriage, induced abortion, childbirth, placental rupture, accidents, or medical procedures carried out during pregnancy.
2. The pregnant woman had a blood transfusion with blood that has the same blood group as the fetus, but not the pregnant woman. This may happen with blood groups that are not commonly tested for in routine transfusion work-up.
3. Naturally occurring antibodies as in ABO incompatibility (see Question 2).

Rh immune globulin (RhlG) contains antibodies to the D antigen in Rh-positive cells. It is given by intramuscular injection to Rh-negative pregnant women during the 28th week of pregnancy, following childbirth, and following any event that could cause fetal-maternal hemorrhage. This immunoglobulin attaches to fetal cells in maternal blood and causes their destruction, inhibiting the pregnant woman’s immune system from producing antibodies to the D antigen and thus protecting against HDFN. Since RhlG contains anti-D antibodies, it interferes with the RBC antibody screen (test code 795) and causes a low-titer positive result if the test is performed shortly after injection.

Because we perform ABO/Rh testing for pregnant patients, we want to ensure that any mother who can become sensitized to the Rh(D) antigen by a Rh(D)-positive fetus is given the opportunity to receive RhIG. Some mothers who are serologically weakly positive for D phenotype (weak D) may still form antibodies to the D antigen. Our test system is designed to identify these mothers with Rh(D) typing discrepancies as D negative, to ensure that they have the opportunity to receive RhIG to prevent future HDFN.^1,2

There are 2 categories of altered Rh(D) expression that can cause typing discrepancies: weak D antigen and partial D antigen. Weak D antigen is observed in individuals who have RHD gene alleles that cause diminished expression (rather than full or no expression) of the D antigen on their red blood cells. Testing of these individuals may variably result in a D+ (Rh positive) or D- (Rh negative) report, depending upon the sensitivity and specificity of commercial Rh(D) typing reagents used in different serological testing methods. Thus, these D variants are often called “weak D+” based on their serological reactivity to anti-D reagents.³

In contrast, up to 4% of patients who have inherited an altered RHD gene possess a partial D antigen or D variant antigen, in which only a portion of the normal D antigen is expressed. Unlike most patients with the weak D phenotype, patients with these partial D phenotypes or D variant phenotypes may form anti-D antibodies if exposed to fetal red blood cells expressing the D antigen. Because routine serologic testing does not differentiate these altered RHD gene subtypes, some women who type as “weak D+” are at risk for developing antibodies that could cause HDFN. Therefore, RhIG may be appropriate for patients with suspected “weak D+” during pregnancy.^3-5

For patients with suspected D variants who wish to avoid the use of RhIg, the Weak RHD Analysis Workup is available from Versiti (Blood Center of Wisconsin) using the Versiti order code 3040. Results from this Workup can be used to classify a patient’s Rh type and determine if RhIg is needed.^3-5