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Rh Immune Globulin
Rh immune globulin (RhIg) is a human plasma–derived product consisting of IgG antibodies to the D antigen. First licensed in 1968, it is used to prevent immunization to the D antigen in D-negative individuals and for the treatment of immune thrombocytopenia (ITP). Prevention of anti-D formation in females of childbearing potential is important because anti-D can cause severe, potentially fatal, hemolytic disease of the fetus and newborn (HDFN). Exposure to as little as 0.03 mL of D-positive erythrocytes can result in sensitization. Perinatal administration of RhIg has decreased the risk of forming anti-D in D-negative women carrying D-positive fetuses from approximately 13% to 0.1% and substantially reduced the risk of HDFN. RhIg can also be effective in treating ITP, a bleeding disorder characterized by immune-mediated platelet destruction, in D-positive patients with intact spleen.
Each dose of RhIg suppresses the immune response for up to a certain amount of D-positive red blood cells (RBCs) ( Table 40.1 ). Measuring the amount of D-positive RBCs within the patient is imprecise, and therefore, the dose of RhIg recommended is greater than the exact dose calculated. Half-life of RhIg is ∼24 days. Moreover, dosing can also vary slightly by manufacturer. Consequently, only general guidelines are provided in the section below.
Table 40.1
RhIg Vial Size and Amount of Whole Blood or RBCs Neutralized
| Vial Size (μg) | IU | Whole Blood (mL) | RBCs (mL) |
|---|---|---|---|
| 50 | 250 | 5 | 2.5 |
| 120 | 600 | 12 | 6 |
| 300 | 1500 | 30 | 15 |
| 1000 | 5000 | 100 | 50 |
The mechanism of action for preventing D immunization is likely due to the anti-D neutralizing the D antigen on RBCs. RhIg is used most commonly in the antenatal/postnatal prevention of anti-D formation in D-negative women and prevention of anti-D formation in D-negative recipients of D-positive RBC-containing products.
Preparation and Administration
RhIg is typically produced from D antigen–negative male donors who are repeatedly exposed to the D antigen. As described for intravenous immunoglobulin (IVIG), the pooled plasma undergoes cold alcohol fractionation, purification, filtration, and infectious disease reduction technologies. There are several RhIg preparations available. Those licensed only for intramuscular (IM) use contain small amounts of IgG aggregates, IgA, IgM, and other proteins that could potentially cause anaphylaxis if administered intravenously. Those products licensed for intravenous (IV) use are prepared by ion exchange chromatography and are purer than the IM preparations. IV injections are generally more comfortable for patients than IM injections if the patient has venous access. IV formulation should be used in ITP patients or other patients with thrombocytopenia to prevent IM bleeding.
Indications to Prevent Anti-D Formation
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