ABO Blood Group System

Importance of ABO system ABO compatibility between donor cell and patient serum is the essential foundation of pretransfusion testing It is the only system with expected antibodies Whether they are IgG or IgM, ABO antibodies can activate complement readily This means that incompatibilities can cause life threatening situations (transfusion reactions)

Biochemical & Genetic Considerations ABO antigens Biochemical & Genetic Considerations

ABO and H Antigen Genetics Genes at three separate loci control the occurrence and location of ABO antigens. The presence or absence of the A, B, and H antigens is controlled by the H and ABO genes. The presence or absence of the ABH antigens on the red blood cell membrane is controlled by the H gene. The presence or absence of the ABH antigens in secretions is indirectly controlled by the Se gene.

ABO Antigen Genetics H gene – H and h alleles. Se gene – Se and se alleles. ABO genes – A, B and O alleles.

H Antigen The H gene codes for an enzyme that adds the sugar fucose to the terminal sugar of a precursor substance (PS) The precursor substance (proteins and lipids) is formed on an oligosaccharide chain (the basic structure)

Type I and Type II Precursors There are two potential precursors substances for ABH antigens Type I and Type II Both are comprised of identical sugars but the linkage of the terminal sugars differs in the two types Type I precursor has a terminal galactose linked to a subterminal N-acetylgluosamine in a 1-3 linkage These same sugars combine in a 1-4 linkage in type II precursor ABH Ags on red cells are derived from Type II chains whereas the ABH Ags in plasma are made from both types I & II precursors

Type II H After fucose is added to Type II chains, the structure is termed Type II H Four kinds of Type II H have been identified H1, H2 are simple straight chain glycolipids Whereas H3 & H4 have branched chains

RBC Precursor Structure Glucose Galactose N acetylglucosamine Precursor Substance (stays the same) RBC

Formation of the H antigen Glucose Galactose N-acetylglucosamine H antigen RBC Fucose H enzyme is fucosyltransferase

H antigen The H antigen is the foundation upon which A and B antigens are built A and B genes code for enzymes that add a sugar to the H antigen Immunodominant sugars are present at the terminal ends of the chains and confer the ABO antigen specificity

A and B Antigen The “A” gene codes for an enzyme (transferase) that adds N-acetylgalactosamine to the terminal sugar of the H antigen N-acetylgalactosaminyltransferase The “B” gene codes for an enzyme that adds D-galactose to the terminal sugar of the H antigen D-galactosyltransferase

Formation of the A antigen Glucose Galactose N-acetylglucosamine RBC Fucose N-acetylgalactosamine

Formation of the B antigen Glucose Galactose N-acetylglucosamine RBC Fucose

Genetics The H antigen is found on the RBC when you have the Hh or HH genotype, but NOT from the hh genotype The A antigen is found on the RBC when you have the Hh, HH, and A/A, A/O, or A/B genotypes The B antigen is found on the RBC when you have the Hh, HH, and B/B, B/O, or A/B genotypes

O>A2>B>A2B>A1>A1B H antigen Certain blood types possess more H antigen than others: O>A2>B>A2B>A1>A1B Greatest amount of H Least amount of H

The O allele Why do Group O individuals have more H antigen than the other groups? The O gene is a silent allele. It does not alter the structure of the H substance….that means more H antigen sites.

Fewer H antigen sites A Group O Group A Group O Group A Many H antigen sites Most of the H antigen sites in a Group A individual have been converted to the A antigen

ABO Subgroups ABO subgroups differ in the amount of antigen present on the red blood cell membrane Subgroups have less antigen Subgroups are the result of less effective enzymes. They are not as efficient in converting H antigens to A or B antigens (fewer antigens are present on the RBC) Subgroups of A are more common than subgroups of B

Subgroups of A The 2 principle subgroups of A are: A1 and A2 Both react strongly with reagent anti-A 80% of group A or AB individuals are subgroup A1 20% are A2 and A2B

A2 Phenotype Why is the A2 phenotype important? A2 and A2B individuals may produce an anti-A1 This may cause discrepancies when a crossmatch is done (incompatibility) What’s the difference between the A1 and A2 antigen? It’s quantitative The A2 gene doesn’t convert the H3 & H4 to A very well The result is fewer A2 antigen sites compared to the many A1 antigen sites

A1 and A2 Subgroups A1 A2 4+ Anti-B 900 x103 3+ Anti-B & anti-A1 Anti-A antisera Anti-A1 antisera Anti-H lectin ABO antibodies in serum # of antigen sites per RBC A1 4+ Anti-B 900 x103 A2 3+ Anti-B & anti-A1 250 x103

Other A subgroups There are other additional subgroups of A Aint (intermediate), A3, Ax, Am, Aend, Ael, Abantu A3 red cells cause mixed field agglutination when polyclonal anti-A or anti-A,B is used Mixed field agglutination appears as small agglutinates with a background of unagglutinated RBCs They may contain anti-A1

B Subgroups B subgroups occur less than A subgroups B subgroups are differentiated by the type of reaction with anti-B, anti-A,B, and anti-H B3, Bx, Bm, and Bel

Other ABO conditions Bombay Phenotype (Oh) Inheritance of hh The h gene is an amorph and results in little or no production of L-fucosyltransferase Originally found in Bombay (now Mumbai) Very rare

Bombay The hh causes NO H antigen to be produced Results in RBCs with no H, A, or B antigen (patient types as O) Bombay RBCs are NOT agglutinated with anti-A, anti-B, or anti-H (no antigens present) Bombay serum has strong anti-A, anti-B and anti-H, agglutinating ALL ABO blood groups What blood ABO blood group would you use to transfuse this patient??

ANSWER: Another Bombay Group O RBCs cannot be given because they still have the H antigen You have to transfuse the patient with blood that contains NO H antigen

ABO Blood Group ABO Antibodies

ABO Blood Group System The ABO Blood Group System was the first to be identified and is the most significant for transfusion practice It is the ONLY system that the reciprocal antibodies are consistently and predictably present in the sera of people who have had no exposure to human red cells

Blood Group Systems Most blood group systems (ABO and others) are made up of: An antigen on a red cell and the absence of it’s corresponding antibody in the serum (if you’re A, you don’t have anti-A) If you do NOT have a particular antigen on your red cells then it is possible (when exposed to foreign RBCs) to illicit an immune response that results in the production of the antibody specific for the missing antigen

ABO Remember: The ABO Blood Group System does NOT require the presence of a foreign red blood cell for the production of ABO antibodies ABO antibodies are “non-red blood cell stimulated” probably from environmental exposure and are referred to as “expected antibodies” Titer of ABO Abs is often reduced in elderly and in patients with hypogammaglobulinemia Infants do not produce Abs until 3-6 months of age

ABO antibodies RBC Phenotype Frequency (%) Serum Ab A 43 Anti-B B 9 Anti-A AB 4 -------- O 44 Anti-A,B

ABO antibodies IgM is the predominant antibody in Group A and Group B individuals Anti-A Anti-B IgG (with some IgM) is the predominant antibody in Group O individuals Anti-A,B (with some anti-A and anti-B)

Clinically Significant Transfusion Reactions ABO antibody facts Complement can be activated with ABO antibodies (mostly IgM, some IgG) High titer: react strongly (4+) Anti-A, Anti-B, Anti-A,B Clinically Significant Yes Abs class IgM, less IgG Thermal range 4 - 37 HDNB Transfusion Reactions Extravascular Intravascular

ABO Antibodies Usually present within the first 3-6 months of life Stable by ages 5-6 years Decline in older age & in hypogammaglobulinemia Newborns may passively acquire maternal antibodies (IgG crosses placenta) Remember: most ABO antibodies are IgM, but some are IgG. Mothers can pass those IgG antibodies through placenta to fetus

Nature of antibodies Non-red blood cell stimulated ABO antibodies Antibodies formed as a result of transfusion, etc Usually IgG Active at 37°C Can occur in group O (may occur in group A or B) These antibodies also occur in the other Blood Group Systems

Anti-A,B Found in the serum of group O individuals Reacts with A, B, and AB cells Predominately IgG, with small portions being IgM Anti-A,B is one antibody, it is not a mixture of anti-A and anti-B antibodies

Anti-A1 Anti-A1 Clinically Significant Sometimes Abs class IgM Thermal range 4 - 22 HDNB No Transfusion Reactions Extravascular Intravascular Rare Group O and B individuals contain anti-A in their serum However, the anti-A can be separated into different components: anti-A and anti-A1 Anti-A1 only agglutinates the A1 antigen, not the A2 antigen There is no anti-A2.

Anti-H Auto-Anti-H Clinically Significant No Abs class IgM Thermal range 4 - 15 HDNB Transfusion Reactions Extravascular Intravascular Allo-Anti-H Clinically Significant Yes Abs class IgM, IgG Thermal range 4 - 37 HDNB Transfusion Reactions Extravascular Intravascular

ABO Antigens in Secretions Secretions include body fluids like plasma, saliva, synovial fluid, etc Blood Group Substances are soluble antigens (A, B, and H) that can be found in the secretions. This is controlled by the H and Se genes

Secretor Status The secretor gene consists of 2 alleles (Se and se) The Se gene is responsible for the expression of the H antigen on glycoprotein structures located in body secretions If the Se allele is inherited as SeSe or Sese, the person is called a “secretor” 80% of the population are secretors

Secretors Secretors express soluble forms of the H antigen in secretions that can then be converted to A or B antigens (by the transferases) Individuals who inherit the sese gene are called “nonsecretors” The se allele is an amorph (nothing expressed) sese individuals do not convert antigen precursors to H antigen and has neither soluble H antigen nor soluble A or B antigens in body fluids Secretions include saliva, urine, tears, bile, amniotic fluid, breast milk, exudate, and digestive fluids.

Secretor Status Summary The Se gene codes for the presence of the H antigen in secretions, therefore the presence of A and/or B antigens in the secretions is contingent on the inheritance of the Se gene and the H gene A antigen Se gene (SeSe or Sese) H antigen in secretions and/or B antigen No antigens secreted in saliva or other body fluids se gene (sese)

Secretors (SeSe or Sese): Non-secretors (sese): ABO Group ABH Substances Secretors (SeSe or Sese): A B H +++ + O AB Non-secretors (sese): A, B, O, and AB Sese + h/h (no H antigen)  no antigens in secretions