Blood Groups, Transfusion Immunology & Hemolytic Disease of the Newborn
Antigens on red cells, antibodies in plasma, and what happens when they meet
ABO and Rh antigens
Building ABO antigens. The FUT1 gene product (H transferase) adds fucose to a precursor chain on RBC glycoproteins and glycolipids, forming H antigen. Codominant alleles of the ABO gene on chromosome 9 then decide the final sugar: the A transferase adds N-acetylgalactosamine, the B transferase adds galactose, and group O, lacking a functional enzyme, keeps plain H. These carbohydrate antigens also coat many non-red cells. In the rare Bombay phenotype (hh) there is no H antigen and therefore no A or B; the person types as O but makes anti-H and can receive only Bombay blood.
Natural antibodies. By about 3–6 months of age, infants make antibodies to the A or B antigens they lack, triggered by cross-reacting carbohydrates on gut bacteria. These isohemagglutinins are mainly IgM, which fixes complement strongly, so a mismatched unit is lysed intravascularly within minutes; IgM does not cross the placenta. Group O individuals also make some IgG anti-A,B, which can.
Rh(D). The D antigen is a transmembrane protein encoded by RHD. About 15% of White individuals (fewer African and Asian individuals) are D-negative. Unlike ABO, there is no natural anti-D: antibody forms only after exposure through transfusion or pregnancy, and it is IgG, which crosses the placenta and causes extravascular (splenic) hemolysis.
| Group | RBC antigens | Plasma antibodies | Can receive RBCs from | Can receive plasma from |
|---|---|---|---|---|
| O | None (H only) | Anti-A, anti-B | O | O, A, B, AB |
| A | A | Anti-B | A, O | A, AB |
| B | B | Anti-A | B, O | B, AB |
| AB | A and B | None | AB, A, B, O | AB |
Universal donors and recipients. Group O, D-negative RBCs carry no A, B or D, so they are the universal RBC donor; uncrossmatched emergency RBCs are group O, and D-negative for girls and women of childbearing age. Group AB, D-positive patients make no anti-A or anti-B and are universal RBC recipients. For plasma the logic reverses: AB plasma contains no anti-A or anti-B and is the universal plasma donor, while group O patients, whose RBCs carry neither antigen, are universal plasma recipients.
Antiglobulin (Coombs) testing
Direct antiglobulin test (DAT). The patient's washed RBCs are mixed with anti-human IgG and anti-C3; agglutination means the cells were already coated in vivo. It is positive in warm autoimmune hemolytic anemia, drug-induced immune hemolysis, hemolytic disease of the newborn (on the infant's cells) and hemolytic transfusion reactions.
Indirect antiglobulin test (IAT). The patient's serum is first incubated with reagent RBCs of known antigen type, then antiglobulin is added. It detects free antibody in serum, so it powers the pretransfusion antibody screen, the crossmatch and prenatal screening for anti-D.
Direct = antibody on the patient's own red cells; indirect = antibody floating in the patient's serum.
Hemolytic disease of the fetus and newborn
| Feature | ABO HDFN | Rh (anti-D) HDFN |
|---|---|---|
| Setting | Group O mother, A or B baby | D-negative mother, D-positive baby |
| Antibody | Naturally present IgG anti-A,B | IgG anti-D after prior sensitization |
| First pregnancy | Can be affected | Usually spared |
| Severity | Mild: jaundice within 24 h, weakly positive DAT, spherocytes | Severe: anemia, hydrops fetalis, kernicterus |
| Management | Phototherapy | Prevention with anti-D; intrauterine transfusion, exchange transfusion |
ABO disease. It is now more common than Rh disease but mild, because A and B antigens are weakly expressed on fetal RBCs and much of the antibody is absorbed by other fetal tissues.
Rh disease. A D-negative mother is sensitized by fetomaternal hemorrhage from a D-positive fetus at delivery, miscarriage, abortion, ectopic pregnancy, abdominal trauma or amniocentesis. In a later pregnancy her IgG anti-D crosses the placenta and destroys fetal RBCs, producing anemia, extramedullary hematopoiesis with hepatosplenomegaly, high-output heart failure and hydrops fetalis (skin edema, ascites, pleural and pericardial effusions). Before birth the placenta hands fetal bilirubin to the mother for clearance, but after birth unconjugated bilirubin rises quickly and can cause kernicterus. Fetal anemia is followed with middle cerebral artery Doppler peak systolic velocity and treated with intrauterine transfusion.
Anti-D immune globulin. Given to D-negative women who have not yet formed anti-D, it clears D-positive fetal cells before the maternal immune system responds; it is useless once anti-D exists.
- 28 weeksroutine antenatal dose after a negative antibody screen
- Sensitizing eventsmiscarriage, abortion, ectopic pregnancy, amniocentesis, bleeding, abdominal trauma
- Within 72 hours of deliveryif the newborn is D-positive
- Size the doserosette screen, then Kleihauer-Betke; a 300 μg dose covers about 30 mL of fetal whole blood
Kleihauer-Betke test. Acid elution strips adult hemoglobin, leaving maternal cells as pale ghosts, while HbF-rich fetal cells resist the acid and stain pink; counting them estimates the volume of fetomaternal hemorrhage.
Transfusion reactions
- Acute hemolytic (minutes): usually a clerical ABO error. Recipient IgM anti-A/B fix complement on donor cells (type II hypersensitivity) → intravascular hemolysis with fever, chills, flank pain, hypotension, hemoglobinuria, DIC and acute kidney injury; DAT positive, haptoglobin low. Stop the transfusion, support blood pressure and urine output, and recheck patient and unit identity.
- Febrile nonhemolytic (within hours): the most common reaction, caused by cytokines released from donor leukocytes during storage (and recipient antibodies to donor WBC antigens). Exclude hemolysis and bacterial contamination, give antipyretics; prestorage leukoreduction prevents it.
- Allergic and anaphylactic (minutes): urticaria is a type I reaction to donor plasma proteins, treated with an antihistamine. Anaphylaxis with hypotension, bronchospasm and angioedema strikes IgA-deficient recipients whose anti-IgA antibodies meet donor IgA; give epinephrine, then use washed products or IgA-deficient donors.
- TRALI (within 6 hours): donor anti-HLA or anti-neutrophil antibodies, often from multiparous female donors, activate recipient neutrophils sequestered in the lung → noncardiogenic pulmonary edema with hypoxemia, fever and hypotension, normal BNP and no JVD. Care is supportive.
- TACO (within hours): too much volume too fast in elderly patients or those with heart or kidney disease → hydrostatic edema with hypertension, JVD, S3 and high BNP; slow the rate and give diuretics.
- Delayed hemolytic (3–10 days): a recipient sensitized by earlier transfusion or pregnancy has antibody below detection, classically anti-Kidd (Jk), which fades quickly between exposures, or anti-Rh, Kell or Duffy. Re-exposure triggers an anamnestic IgG response → falling hemoglobin, mild jaundice, extravascular hemolysis and a newly positive DAT and antibody screen; usually self-limited.
- TA-GVHD (about 1–2 weeks): viable donor T cells engraft in a host who cannot reject them and attack skin, gut, liver and marrow → rash, diarrhea, hepatitis and pancytopenia; mortality exceeds 90%.
Choosing modified products
Leukoreduction filters out WBCs, reducing febrile reactions, HLA alloimmunization (a cause of platelet refractoriness) and CMV transmission. Washing removes plasma, for IgA-deficient recipients with anti-IgA and for severe recurrent allergic reactions. Irradiation stops donor T-cell proliferation and is the only reliable prevention of TA-GVHD; it is required for patients with severe cellular immunodeficiency, stem cell transplant recipients, those treated with purine analogs such as fludarabine, Hodgkin lymphoma, intrauterine and neonatal transfusions, and directed donations from relatives, whose shared HLA haplotype lets donor T cells escape rejection.
Leukoreduction does not prevent TA-GVHD: only irradiation reliably blocks donor T-cell proliferation.