Week 2 ABO blood group system

ABO blood group system

• react at body temp (37C)

• responsible for severe hemolytic transfusion reactions

• able to activate classical complement cascade

Pre-transfusion testing

• ABO typing is one of the first testing done before a transfusion

  • recipient and donor must be typed

• to avoid giving the wrong blood type due to mistyped or incorrectly drawn specimen

  • compare current blood type to historical

    • if none then draw 2nd

    • another tech re-types current

grading agglutination in tube testing

• graded on a scale of 0-4+

Grading agglutination in Gel testing

ABO testing - forward type

• patient RBCs mixed with reagent antisera (antibody)

• determines which antigens present on patient RBC

ABO testing - reverse type

• patient plasma/serum mixed with reagent RBC (A1 cells and B cells)

• Determines which antibody

  present in patient serum

ABO testing chart

ABO discrepancy

• why test the forward and reverse group?

  • can have discrepancy between results

• don’t match

• ABO cannot be interpreted until discrepancy is resolved

• transfuse group O if emergency and can’t wait for resolution

ABO antibodies

• begin to develop at 4 months

  • production is stimulated when exposed to substances in nature that are chemically similar to A and B antigen

  • stays constant after peaking 5-10

  • titer could decrease in elderly

• usually IgM antibodies

• some O types produce an additional antibody (anti-A,B) that has dual specificity for A and B antigen (usually IgG)

• preferentially agglutinates RBC at room temp or colder

• activate complement at body temp

ABO antigens

• newborn RBCs do have ABO antigens

  • not fully developed

  • half the amt of antigens on newborn RBCs

• by 2-4 years, antigen completely developed

  • # of antigen sites stays constant once developed

Genetics of ABO antigens

• A, B, O are major alleles

  • located on chromosome 9

• A/B are co-dominant

• O allele is an amorph

  • doesn’t produce a detectable antigen

• genes located on chromosome 19 are important in A and B antigen production

  • gene for H antigen

biochemistry of ABO antigens

• A and B antigens are the last sugars at the end of a long chain of sugars stemming from RBC membrane

• H antigen is the precursor to A and B antigens

  • last sugar added to the oligosaccharides before A and B antigen

  • A and B antigen production depend on the presence of H antigen

Formation of H antigen

• greater than 99.9% of population of H antigen gene

• H gene (HH or Hh) encodes L-fructosyltransferase to add L-fucose molecule to type-2-precursor substance on RBC

• Type O individuals will have highest amt of H antigen present

Bombay phenotype (Oh)

• hh genotype

• H antigen is not formed

• looks like an O phenotype even if A and/or B genes are inherited

• will develop Anti-H whereas typical O have H antigen

  • IgM clinically significant antibody that binds complement and reacts at 37C

• Lacks H antigen

Ulex europaeus

• lectin used for testing

• O + Anti-H = +

• Bombay + Anti-H = negative

Formation of A antigen

• A gene (AA/AO) encodes N-acetylglucosaminyl transferase to add a N-acetyl-D-galactosamine molecule in to H antigen

  • The N-acetyl-D-galactosamine with the H antigen is the A The N-acetyl-D-galactosamine with the H antigen is the A antigenantigen

  • Need A and H genes

• strong and converts most H antigen to A

subgroups of A antigen

• often discovered if there is an ABO discrepancy

  • don’t need to differentiate unless

• Most prominent A1 + A2

  • make up 99% of A individuals

  • A1 - 80%

A1 gene

• causes the N-acetylgalactosaminyl transferase to be produced in high amt

• convert 4x the number of antigen sites compared to A2

A2 gene

• differs from A1 by the ability to develop anti-A1

  • 1-8% A2 and 22-35% A2B develop anti-A1

Less common subgroups

• A3, Am, Ax, Ael

• could be a neg result with anti-A antisera

• some Anti-A,B antisera reagents are better at picking up weak subgroups

• increased expression of H antigen as the A antigen expression decreases

• A3 can have mixed field reaction with anti-A human source antisera used

• Ax usually agglutinates with anti-A,B weakly and no reaction with anti-A

• absorption/elution are techniques used to show the presence of weaker subgroups on RBC

Anti-A1 in subgroup

• IgM naturally occurring antibody

  • tends to react under 37C

• forms in patients w/o A1 antigen

• Lectin Dolichos biflorus will only bind A1 antigen

  • subgroups other than A1 won’t agglutinate

Formation of B antigen

• B gene (BB/BO) encodes D-galactosyltransferase to add a D-galactose molecule to H antigen

  • D-galactose with H antigen is B antigen

  • need B and H genes

• converts less H antigen to B antigen compared to A

Formation of Group AB

• Group AB contain the A and B genes that encode N-acetylgalactosaminyl transferase and D-galactosyltransferase to add a N-acetyl-D galactosamine and a D-galactose molecule to H antigen

  • Need the A, B and H genes

• N-acetylgalactosaminyl transferase and D-galactosyltransferase both converting H antigen to A and B

  • Has the least amount of H antigen of the ABO blood types

biochemistry of ABO antigens

H antigen concentration

• adding A and B antigens to RBC membrane decreases the amt of H antigen present

  • inversely proportionate relationship

  • effects ability to detect H antigen

• Anti-H can develop occasionally in patients with low amt of H antigen

  • A1 A1B

Secretion of ABH

• secretor gene (Se and se alleles) found on chromosome 19

  • responsible for ABH soluble substances found in body fluids

  • 80% of population are secretors (SeSe or Sese)

  • encodes L-fucosyltransferase to add L-fucose molecule to type-1-precursor substance

    • ^ found in secretory tissues

secretion of ABH and ABO antigens