Blood Bank / Immunohematology
ABO and Rh fundamentals
ABO antigens are carbohydrates; Rh, Kell, Duffy, and Kidd antigens are proteins. Forward (cell) typing detects RBC antigens with reagent antisera; reverse (serum) typing detects plasma antibodies against A1 and B cells. Group O plasma contains anti-A, anti-B, and anti-A,B. D is the most immunogenic Rh antigen, and weak D or partial D variants may type differently depending on antisera and platform.
Antibody detection
The antibody screen tests patient serum against 2–3 reagent RBC sets at immediate spin, 37°C, and AHG. IgG antibodies reacting at 37°C/AHG (Rh, Kell, Duffy, Kidd, MNS) are clinically significant; IgM antibodies (anti-I, anti-P1, Lewis) usually are not. Antibody identification uses panel cells, including homozygous cells, because some antibodies show dosage.
Crossmatch and reactions
Immediate-spin crossmatch confirms ABO compatibility; AHG crossmatch detects IgG. The DAT detects in vivo coating and is positive in autoimmune hemolysis, delayed hemolytic reactions, and HDFN. Acute hemolytic transfusion reactions (fever, chills, hemoglobinuria, DIC) usually reflect clerical or ABO errors; delayed reactions appear 5–14 days later with falling hemoglobin and a positive DAT.
Traps and rule of thumb
- Infants under ~4 months have no anti-A/anti-B; interpret only forward typing together with the mother's type.
- A2 and A2B patients may form anti-A1, producing a reverse-typing discrepancy.
- Bombay (Oh) phenotype lacks H antigen and contains anti-H, reacting with all ordinary group O cells.
Rule of thumb: never transfuse on a single ABO discrepancy - resolve the discrepancy first.