Human blood is classified as Rh positive or Rh negative. This relates to the presence or the absence of the D antigen on the red cell membrane. The D antigen (also called Rh₁ [D]) is, after the A and B antigens, the next most important antigen in transfusion practice.

The Rh system is composed of antigens tested for in conjunction with the ABO group. Rh antigens (there are more than 50) are determined by two closely linked genes on chromosome 1. Rh₁ (D) antigen is often the only factor tested for. When this factor is absent, further testing is then done on women of childbearing age to identify if there is Rh₁ (D) antigen present in smaller amounts. This test is called weak D (formally known as D testing). Rh-negative individuals may develop antibodies against Rh-positive antigens if they are challenged through a transfusion of Rh-positive blood or through a fetomaternal bleed from an Rh-positive fetus. See Table 8.16.

White:

• 85% or 0.85 Rh positive (have the Rh(O) antigen)

• 15% or 0.15 Rh negative (lack the Rh(O) antigen)

Black:

• 90% or 0.90 Rh positive (have the Rh(O) antigen)

• 10% or 0.10 Rh negative (lack the Rh(O) antigen)

1. Blood Rh typing must be done for the following reasons:

   1. Rh-positive blood administered to an Rh-negative person may sensitize the person to form anti-D (Rh₁).

   2. Rh₁ (D)-positive blood administered to a recipient having serum anti-D (Rh₁) could be fatal.

2. Identify Rh immune globulin (RhIG) candidates. RhIG is a concentrated solution of IgG anti-D (Rh₁) derived from human plasma. A 1-mL dose of RhIG contains 300 μg and is sufficient to counteract the immunizing effects of 15 mL of packed red cells or 30 mL of whole blood.

   1. Rh-negative pregnant women with Rh-positive partners may carry Rh-positive fetuses. Fetal cells may cross the placenta to the mother and cause production of antibodies in the maternal blood. The maternal antibody, in turn, may cross through the placenta into the fetal circulation and cause destruction of fetal blood cells. This condition, called hemolytic disease of the newborn (formerly called erythroblastosis fetalis), may cause reactions that range from anemia (slight or severe) to fetal death in utero. This condition may be prevented if an Rh-negative pregnant woman receives an RhIG dose antepartum at 28 weeks’ gestation and a postpartum injection of RhIG shortly after delivery of an RhD (Rh₁)-positive infant. Postpartum Rh immunization can occur despite an injection of RhIG if >30 mL of fetal blood enters the maternal circulation. The American Association of Blood Banks recommends that a postpartum blood specimen of all RhD (Rh₁)-negative women (i.e., those at risk for immunization) be examined to detect a fetal maternal hemorrhage of >30 mL.

   2. Rh typing and evaluation for RhIG must also be done for patients who have had abortions, miscarriages, injuries, and amniocentesis.

3. Observe standard precautions.

1. The significance of Rh antigens is based on their capacity to immunize as a result of receiving a transfusion or becoming pregnant. The Rh₁ (D) antigen is by far the most antigenic; the other Rh antigens are much less likely to produce isoimmunization. The following general conditions must be met for immunization to Rh antigens to occur:

   1. The Rh blood antigen must be absent in the immunized person.

   2. The Rh blood antigen must be present in the immunizing blood.

   3. The blood antigen must be of sufficient antigenic strength to produce a reaction.

   4. The amount of incompatible blood must be large enough to induce antibody formation.

   5. Factors other than Rh₁ (D) may induce formation of antibodies in Rh-positive persons if the preceding conditions are met.

2. Antibodies for Rh₂ (C) are frequently found together with anti-Rh₁ (D) antibodies in the Rh-negative pregnant woman whose fetus or child is type Rh positive and possesses both antigens.

3. With exceedingly rare exceptions, Rh antibodies do not form unless preceded by antigenic stimulation, as occurs with the following conditions:

   1. Pregnancy and abortions

   2. Blood transfusions

   3. Deliberate immunization, most commonly of repeated IV injections of blood for the purpose of harvesting a given Rh antibody

Pretest Patient Care

1. Explain purpose and procedure of the typing.

2. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.