Clinical meaning
Blood typing is based on the presence or absence of specific antigens on the surface of red blood cells (erythrocytes) and corresponding antibodies in the plasma. The ABO blood group system is the most clinically significant because ABO antibodies are naturally occurring (present without prior sensitization) and can cause fatal intravascular hemolysis if incompatible blood is transfused. The ABO system is determined by carbohydrate antigens attached to glycoproteins and glycolipids on the red blood cell membrane. The H antigen is the precursor molecule present on all red blood cells. The A gene encodes an enzyme (N-acetylgalactosamine transferase) that adds N-acetylgalactosamine to the H antigen, creating the A antigen. The B gene encodes an enzyme (galactose transferase) that adds galactose to the H antigen, creating the B antigen. Individuals with type O blood have neither A nor B transferase enzymes, so the H antigen remains unmodified. The critical immunological principle is that individuals develop antibodies against ABO antigens they LACK: type A individuals have anti-B antibodies (IgM), type B individuals have anti-A antibodies, type AB individuals have neither antibody (universal plasma recipient), and type O individuals have both anti-A and anti-B antibodies (universal red cell donor but universal plasma recipient is type AB). These naturally occurring IgM antibodies can activate the complement cascade and cause immediate intravascular hemolysis upon contact with incompatible red blood cells. The Rh (Rhesus) blood group system involves approximately 50 antigens, but the D antigen is the most immunogenic and clinically significant. Individuals are classified as Rh-positive (D antigen present, approximately 85% of the population) or Rh-negative (D antigen absent). Unlike ABO antibodies, anti-D antibodies are NOT naturally occurring -- they develop only after exposure to D-positive red blood cells through transfusion or pregnancy. This is why Rh-negative mothers can develop anti-D antibodies after carrying an Rh-positive fetus (Rh sensitization), leading to hemolytic disease of the fetus and newborn (HDFN/erythroblastosis fetalis) in subsequent Rh-positive pregnancies. RhoGAM (Rh immune globulin) prevents sensitization by binding to fetal D-positive red blood cells in the maternal circulation before the mother's immune system can mount a response. Crossmatching is the final compatibility test performed before transfusion: donor red blood cells are mixed with recipient serum in the laboratory. If the recipient has antibodies against donor red blood cell antigens, agglutination (clumping) or hemolysis will occur, indicating incompatibility. A compatible crossmatch shows no agglutination or hemolysis, confirming that the specific donor unit is safe for the specific recipient. The type and screen (T&S) identifies the patient's ABO/Rh type and screens for unexpected alloantibodies, while the crossmatch tests compatibility between a specific donor unit and the patient.