Human Blood Group Antigens and Antibodies

Published on 04/03/2015 by admin

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Chapter 50 Human Blood Group Antigens and Antibodies

Connie M. Westhoff, Jill R. Storry, Beth H. Shaz

Table 50-1 Blood Group Systems, Antigens, Expression, and Disease Associations

IgSF, Immunoglobulin super family; ISGN, International Society for Gene Nomenclature; type I, protein with a single pass through the RBC lipid bilayer with its amino terminus to the outside of the cell; type II, protein with a single pass through the RBC lipid bilayer with its amino terminus to the inside of the cell.

Table 50-2 Uses of DNA-Based Genotyping Assays for Transfusion Medicine

Type patients who have been recently transfused

Type RBCs coated with immunoglobulin

Type patients with AIHA (to select antigen-negative RBCs for transfusion and absorption of autoantibodies when searching for under-lying alloantibodies)

Type RBCs when commercial antisera are not available

Type for numerous blood group antigens in a single assay

Identify weak D and partial D (to determine candidate for Rh immune globulin or avoid use of limited Rh-negative donor supply)

Resolve blood group typing discrepancies

Determine paternal zygosity for RHD and HPA

Type fetus to determine risk for HDFN or NAIT

Figure 50-1 MODEL OF BLOOD GROUP PROTEINS IN THE RED BLOOD CELL (RBC) MEMBRANE.

Schematic representation of the RBC molecules that carry blood group antigens and the predicted structure. These include carbohydrates, single and multipass proteins, and GPI-linked proteins in the RBC membrane.

Indirect Antiglobulin Test and Direct Antiglobulin Test

The indirect antiglobulin test (IAT) is used to detect alloantibodies in patient sera, or RBCs coated with antibody in vitro following incubation at 37° C, for example, antibody screening and identification, antigen typing, and crossmatching with donor RBCs. After incubation, unbound antibodies are removed from the RBCs by washing with saline, and an antiglobulin reagent containing either antihuman IgG or a mixture of antihuman IgG and monoclonal antihuman complement is added. Agglutination of all cells suggests the presence of an antibody to a high-prevalence antigen or the presence of an autoantibody; differential reactivity suggests the presence of antibodies to one or more specific RBC antigens.

The direct antiglobulin test (DAT) is used to detect the presence of antibody or complement (or both) on the surface of RBCs in vivo such as autoantibodies coating the patient’s cells in warm autoimmune hemolytic anemia, cold hemagglutinin disease, or alloantibodies coating the patient’s cells in immediate or delayed transfusion reactions or hemolytic disease of the fetus and newborn. Patient RBCs obtained in ethylenediaminetetraacetic acid (EDTA) are washed with saline and then incubated with a commercial antiglobulin reagent containing antihuman IgG or antihuman complement or a mixture of the two. Antiglobulin reagents containing anti-IgM or anti-IgA are available in specialized centers to detect coating of RBCs in vivo by antibodies of these isotypes.

Table 50-3 Characteristics of Some Blood Group Alloantibodies (Listed in Approximate Order of Clinical Significance)

HDFN, Hemolytic disease of the fetus and newborn.

* Most examples of these antibodies are both IgM and IgG.

† Seldom hemolysis of fetal cells but high incidence of recurrent spontaneous abortions.

Rh Immune Globulin

RhIG is a human plasma–derived hyperimmunoglobulin product consisting of IgG antibodies to D antigen that is administered to D-negative pregnant women who are at risk for D sensitization. RhIG is administered (a) at 28 weeks gestational age, (b) when there is a risk for fetal maternal hemorrhage through amniocentesis, trauma, or other procedures, and (c) postpartum in the case of a known or potential D-positive newborn or fetus. RhIG is sometimes administered outside of pregnancy to D-negative patients who receive D-positive blood products, most commonly platelet products. This is primarily considered for females of childbearing potential when the formation of anti-D has serious consequences. (Because the risk for D alloimmunization from platelet transfusion is less than 4%, the majority of D-incompatible platelets are given without RhIG administration.) RhIG in significantly higher doses is used to treat immune thrombocytopenic purpura (ITP) in patients who are D-positive and have not been spleenectomized.

For prevention of D-sensitization in the United States, 300 mcg are routinely administered, but dosing is increased if there is evidence of large fetal-maternal hemorrhage (300 mcg for every 15 mL of RBC exposure). The dose is calculated based on the estimated volume of D-positive RBCs from Kleihauer-Betke or flow cytometry testing. RhIG should be given within 72 hours, which was the time period for the original studies, but should not be withheld if not administered within this time period. Adverse events to low doses used to prevent D immunization include fever, chills, and pain at the injection site. Rarely, hypersensitivity reactions are noted. RhIG doses used to treat ITP are substantial: 50 mcg/kg for hemoglobin values ≥10 g/dL and 25 to 40 mcg/kg when hemoglobin is 8 to 10 g/dL. Adverse events include possible anemia, hemolysis, disseminated intravascular coagulopathy, and rarely, death.

Table 50-4 Approaches to Supplying Red Blood Cell Products to Prevent Alloimmunization in Patients with Sickle Cell Disease or Other Transfusion-Dependent Anemia

Phenotype or genotype for clinically significant antigens before transfusion

Provide phenotype-matched blood for C, E, and K prophylactically

Provide blood negative for the major antigens that the patient lacks after the patient makes an antibody

Table 50-5 Prevalence of the principal Rh Haplotypes

Weak or Variable D Typing

Clinically significant D sensitization potentially results in a pregnancy with a fetus at risk for hemolytic disease of the fetus and newborn and hemolytic transfusion reactions if transfused with D-positive RBCs. Individuals with RBCs that express a partial D antigen are at risk for D-sensitization, whereas those with weak D antigen are usually not at risk. These cannot be distinguished by serologic reactivity, because either may present as weak or moderately positive or give variable results with anti-D reagents. Particularly in the prenatal setting, RHD genotyping should be done to distinguish weak D from partial D. Women with weak D expression are not at risk for clinically significant D sensitization and therefore are not candidates for RhIG prophylaxis. In contrast, individuals with partial D lack D epitopes and have produced clinically significant anti-D and should receive RhIG prophylaxis. RHD genotyping avoids unnecessary treatment with RhIG in women with weak D antigen.

Transfusion Management of Patients With Warm Autoimmune Hemolytic Anemia

Patients with warm autoimmune hemolytic anemia may present with jaundice, fatigue, and anemia, or they may show no overt clinical manifestations. The antibody screen and antibody identification panel will show all RBCs positive (panagglutinin) with anti-IgG in the indirect antiglobulin test. The autocontrol (patient’s own plasma and RBCs) will also be positive.

History: A transfusion history should be obtained to differentiate these results from a hemolytic transfusion reaction or hemolysis due to an alloantibody.

DAT: A direct antiglobulin test should be performed with anti-IgG and -C3. In clinically significant hemolysis, the DAT is usually strongly positive.

Eluate: If patient has been recently transfused (3-4 months), an eluate should be prepared from the patient cells to remove the antibody(ies); the eluate should be tested to determine specificity. The eluate is usually reactive with all cells when tested by the IAT with anti-IgG.

Phenotype: Type the patient’s RBCs for minor blood group antigens (Cc, Ee, K, Jka/b, Fya/b, Ss) if the patient has not been recently transfused. When possible, IgM typing reagents are used because the patient’s own antibody-coated RBCs may result in false-positive typing. Some laboratories are able to remove the IgG from the RBCs and perform a phenotype. Alternatively, genotyping for minor blood group antigens including Do^a/b antigens (there is no serologic reagent) can be performed.

Adsorption: Adsorb the serum autoantibody onto the patient’s own RBCs to test for underlying alloantibody if the patient has not been recently transfused (3-4 months). If the patient has been recently transfused or if the low hematocrit results in insufficient autologous RBCs, perform alloadsorption with well-characterized RBCs (usually three with known antigen profiles). Test the adsorbed serum for underlying alloantibodies.

Crossmatch: Perform with neat and with adsorbed plasma. Crossmatch performed with neat plasma will usually be incompatible.

Communication: Inform ordering physician of reactivity and of delay in receiving crossmatched RBCs. Provide emergency-release RBCs if patient’s clinical situation warrants. Inform the physician that the patient may hemolyze transfused RBCs similar to hemolysis of his or her own RBCs.

Transfusion: Consider providing RBC units negative for minor antigens that the patient also lacks (consider matching for Cc, Ee, K, Jka/b, Fya/b, Ss). This potentially enables RBC units to be available before completion of the antibody identification testing. Transfusion with antigen matched units potentially allows continued transfusion without need for auto- or alloadsorption unless signs and symptoms of RBC destruction occur or there is a change in reactivity in antibody screening or the DAT.

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