Immunohematology and Genetics: Blood Groups, Antibody Detection, and Compatibility Testing
Core concepts
Blood group antigens are inherited, mostly single-nucleotide polymorphisms producing glycoproteins or glycolipids. The ABO system is the only one with naturally occurring (preformed) IgM antibodies, which is why ABO compatibility is mandatory. Rh(D) is highly immunogenic; weak D and partial D matter for donor versus patient typing. Other clinically significant systems: Kell (K), Duffy (Fy), Kidd (Jk), MNS, and Lewis.
Antibody detection uses the indirect antiglobulin test (IAT): patient serum against reagent red cells, with anti-IgG reagent and check cells. Antibody identification panels use pattern matching-reactions that disappear with enzyme-treated cells suggest Rh or Kidd; reactions enhanced by enzymes suggest Rh, Kidd, Lewis, P1. Dosage effects (homozygous vs heterozygous cells) are a classic trap.
Compatibility testing: ABO/Rh type, antibody screen, and crossmatch. Immediate spin crossmatch detects ABO incompatibility; full AHG crossmatch is required when the screen is positive. Electronic crossmatch is permitted only if the antibody screen is negative and the patient has a valid historical type.
Traps
- Antibody screen negative does not exclude all clinically significant antibodies-screen sensitivity and dosage matter.
- A positive DAT does not equal hemolysis; correlate with eluate and clinical picture.
- Rh immune globulin: 300 µg covers ~30 mL fetal whole blood; dose by Kleihauer-Betke or flow.
Worked example
A patient's serum reacts with all panel cells except those lacking Jk(a). Enzyme-treated cells lose the reaction. This pattern suggests anti-Jk(a); select Jk(a)-negative units and perform AHG crossmatch.