ImmunoHema

• ABO Blood Group System:

  • Discovered by Karl Landsteiner in 1901
  • Most important blood group in transfusion and transplant medicine
  • Individuals have antibodies in their serum to antigens absent from their RBCs without prior exposure
  • Wrong ABO group can cause death in Hemolytic Transfusion Reactions (HTR) fatalities
• Inheritance of ABO Blood Groups:

  • Follows Mendelian genetics of codominance
  • ABO gene located on chromosome 9
  • A and B genes are codominant, O gene is an amorph
  • A, B, AB, and O are phenotypes; AO, AA, BO, BB, AB, OO are genotypes
• Formation of A, B, and H RBC Antigens:

  • ABO, Hh, and Se have separate loci for ABH antigens formation
  • ABH antigens originate from glycan (paragloboside)
  • ABH antigens on RBCs are made on oligosaccharide chains of type 2 precursor substance
• Hh and ABO Genes:

  • H gene produces α-2-L-fucosyltransferase for L-fucose transfer
  • A gene (AA or AO) codes for α-3-N-acetylgalactosaminyltransferase
  • B gene (BB or BO) codes for α-3-D-galactosyltransferase
• Soluble A, B, and H Antigens:

  • Sese or SeSe (secretor genes) regulate ABH soluble antigens in secretions
  • FUT 2 (Se) gene modifies type 1 precursor substance to form H substance
  • Secretors (SeSe or Sese) secrete glycoproteins carrying A and H antigens
• ABO Subgroups: Type A:

  • A subgroups show weaker serologic reactivity with anti-A, anti-B, and anti-A,B reagents
  • A1 (or A1B) is 80%, A2 (or A2B) is 20%
  • A1 gene creates more antigen sites on RBCs compared to A2 gene
• ABO Subgroups:

  • Type A subgroups have characteristics such as a decreased number of A antigen sites per RBC, varying degrees of agglutination by human-A1,B, and increased variability in the detectability of H antigen
  • Weaker A2 subgroups occur infrequently and are most often recognized through an ABO discrepancy (unexpected reactions in the forward and reverse grouping)
• Weak A Subgroups:

  • Very rare and much less frequent than A subgroups
  • Usually recognized by variations in reaction strength using anti-B and anti-A,B
• Weak B Subgroups:

  • Very rare and much less frequent than A subgroups
  • Usually recognized by variations in reaction strength using anti-B and anti-A,B
• ABO Antibodies:

  • Produced and directed against the antigen absent on the RBC surface
  • Mostly IgM, activates complement, and react at room temperature or colder
  • Production is initiated at birth but titers are too low for detection until 3-6 months
  • Peaks at 5-10 years old but declines later in life
• ABO Discrepancies:

  • Occur when unexpected reactions are obtained in the blood typing due to problems with the patient's serum, RBCs, or both
  • Must be resolved prior to reporting a patient or donor ABO group
• ABO Discrepancies due to Technical Errors:

  • Investigate all possible technical and clerical errors
  • Repeat testing using saline RBC suspension
  • Obtain patient details and background including transfusion history
  • Draw a new sample and run the test again
• Categories of ABO Discrepancies:

  • Group I: Associated with unexpected reactions in the reverse grouping due to weakly reacting or missing antibodies
  • Group II: Associated with unexpected reactions in the forward grouping due to weakly reacting or missing antigens
  • Group III: Discrepancies between forward and reverse groupings caused by protein or plasma abnormalities
  • Group IV: Discrepancies between forward and reverse groupings due to miscellaneous problems
• Group I Discrepancy:

  • More common than those in other groups
  • Associated with unexpected reactions in the reverse grouping due to weakly reacting or missing antibodies
  • Common reasons include depressed antibody production, inability to produce ABO antibodies, newborns, elderly, patients with certain medical conditions, and ABO subgroups
• Group II Discrepancy:

  • Associated with unexpected reactions in the forward grouping due to weakly reacting or missing antigens
  • Causes include subgroups of A or B, leukemia, and acquired B phenomenon
• Group III Discrepancy:

  • Discrepancies between forward and reverse groupings caused by protein or plasma abnormalities resulting in rouleaux formation or pseudoagglutination
  • Attributable to elevated levels of globulin, fibrinogen, plasma expanders, and Wharton's jelly in cord blood samples
• Group IV Discrepancy:

  • Discrepancies between forward and reverse groupings due to miscellaneous problems like cold reactive autoantibodies, circulating RBCs of >1 ABO group, unexpected ABO isoagglutinins, and unexpected non-ABO alloantibodies
• type 1 precursor substance: beta 1-3 linkage between terminal galactose and N-acetylglucosamine
• type 2 precursor substance: beta 1-4 linkage between terminal galactose and Nacetylglucosamine
• H gene elicits the production of α-2-L-fucosyltransferase that transfers the sugar L-fucose to an oligosaccharide chain on the terminal galactose of type 2 chains
• H antigen: the precursor structure on which A and B antigens are made
• FUT 1 (H): RBC surface
• FUT 2 (Se): secretion
• nonsecretors: sese genotype
• A2 allele is characterized by a single base substitution at nucleotide 467 and a single base deletion at nucleotide 1060 (1060delC) in exon 7 that substitutions alter the active site of the coding region
• ABO Ab: mostly IgM, activates complement, and react at room temperature or colder
• A major ABO incompatibility occurs when the donor’s red RBCs are incompatible with the recipient’s plasma (e.g., the donor is group B and the recipient is group O with naturally occurring anti-B).
• A minor ABO incompatibility occurs when the donor’s plasma is incompatible with the recipient’s RBCs (e.g., the donor is group O with naturally occurring anti-B and the recipient is group B).
• Chimerism is defined as the presence of two cell populations in a single individual.
• Acquired B phenomenon occurs when the acquired B antigen arises when bacterial enzymes modify the immunodominant blood group A sugar (N-acetyl-Dgalactosamine) into D-galactosamine, which is sufficiently similar to the group B sugar (D-galactose) to cross-react with anti-B antisera.
• Acquired B phenomenon: Its associated conditions are colorectal malignancy, gastrointestinal obstruction, colon cancer and gram-negative sepsis
• lectins: seed extracts that agglutinate human cells with some degree of specificity
• Dolichos biflorus : agglutinates A1 or A1B
• Bandeiraea simplicifolia: agglutinates B cells
• Ulex europaeus: agglutinates O cells (H specificity)