Immunohematology Notes

Applications of IH Tests

  • IH tests are based on antigen-antibody (Ag-Ab) interactions.
  • Antigens (Ag): Molecules that elicit an immune response (proteins, carbohydrates, lipids).
  • Antibodies (Ab): Proteins produced by the immune system to bind specific antigens.
  • Ag-Ab reactions in IH tests:
    • Agglutination: Abs bind to Ags on red cells, causing clumping.
    • Hemolysis: Abs bind to Ags, destroying red cells and releasing hemoglobin.

Clinical Applications of IH Tests

  • HDFN (Hemolytic Disease of the Fetus and Newborn):
    • Antibody screening of mother's blood.
    • Fetal blood sampling to assess anemia.
    • Fetal monitoring.
    • Blood transfusions (intrauterine or neonatal).
  • AIHA (Autoimmune Hemolytic Anemia):
    • Detect autoantibodies.
  • Other Hematologic Disorders: Diagnose and monitor anemias, leukopenia, thrombocytopenia.
  • Infectious Diseases: Detect antibodies to pathogens (e.g., Hepatitis B and C).
  • Cancer: Detect and monitor certain cancers (e.g., lymphoma, leukemia) using flow cytometry or immunohistochemistry.
  • Transplantation: Identify compatible donors and recipients.
  • Compatibility Testing (Pre-transfusion testing):
    • Donor sample testing (ABO, Rh, TTIs, antibody ID).
    • Patient sample testing (ABO, Rh, antibody screening).
    • Crossmatching.

Compatibility Testing

  • Involves:
    • Sample identification, collection, and preparation.
    • Donor sample testing by collecting facility.
    • Confirmation of ABO/Rh by transfusing facility.
    • Patient sample testing (ABO, Rh, antibody screen).
    • Crossmatching.
  • Crossmatching is a component of compatibility testing.
  • Limitations:
    • Cannot guarantee RBC viability.
    • Cannot totally eliminate transfusion reactions.

Crossmatching Types

  • Serologic Crossmatch:
    • Major: Donor RBCs + Recipient Serum
    • Minor: Recipient RBCs + Donor Serum
    • Phases: Saline (RT & 37°C), Albumin, Antiglobulin
  • Computer Crossmatch: Computer system checks for ABO/Rh compatibility based on donor/recipient records.
  • Emergency Crossmatch: Abbreviated or incomplete crossmatch during emergencies.

Antibody Screening

  • Uses group O reagent red cells (2-5% suspension).
  • Detects dosage (homozygous genotype).
  • Antigens not weakened.

Panel Cell Testing

  • Uses 11-20 group O RBC reagents with various antigen expressions.
  • Antigram provides antigen profile of RBC reagents.
  • Process:
    • Mix patient serum with panel cells.
    • Incubate in different phases: LISS 37C, LISS AGT, enzyme treatment (Ficin).
    • Note reactions (0, +, w).
    • Cancel out negative reactions to identify potential antibodies.

Antigram Interpretation

  1. Identify reagent cells with negative results and cancel them out.
  2. Cancel out antigens present in cells with negative reactions.
  3. Analyze remaining reactions to identify the antibody.
  4. Consider the pattern of reactions and compare with known antibody characteristics.

Exceptions: Polyagglutination

  • Red cells are agglutinated by most human sera.
  • Usually non-reactive with autologous serum.
  • Caused by altered red cell surface antigens (cryptantigens exposed).
  • Types:
    • Microbial associated (transient).
    • Non-microbial associated (permanent).
  • Apparent in minor crossmatch.

Polyagglutination Confirmation

  • Test RBCs with cord blood sera and normal group AB adult sera.
  • Classify using enzymes and lectins.
  • To address polyagglutination you can use:
    • Enzymes, aged sera, dilution, sulfhydryl compounds, adsorption & elution.
  • Associated with hemolytic anemia/HTR, hemolytic uremic syndrome, cancer, leukemia.

Other Considerations & Limitations

  • ABO-incompatible kidney transplant: IH tests may not predict rejection.
  • AIHA: Autoantibodies can interfere with testing.
  • Weak or missing antigens: Difficult to detect corresponding antibodies.
  • Technical limitations: Specimen handling, instrumentation, and operator variability.
  • Pre-analytical, analytical, and post-analytical errors.