D i.e. AO- Highly immunogenic ABO blood group antigen is due to complex oligosaccharide (glycoprotein) found on RBC membrane. It is inherited as medelian autosomal dominantsQ.- ABO blood group system remains the most impoant blood group system in clinical practice because ABO antibodies are invariably present in plasma when person's RBC's lack the corresponding antigen. And because of these preformed antibodies the reaction is inunediate.Genotype of a person with A blood group may either be AA (homozygous) or AO (heterozygous). The genotype in B blood group may be BB (Hamozygous) or BO (heterozygous); in AB blood group AB and in blood group O the genotype is OO.- Highly immunogenic ABO blood group antigen is due to complex oligosaccharide (glycoprotein) found on RBC membrane. It is inherited as medelian autosomal dominantsdeg.- Alleles of ABO gene are located on long arm of chromosome 9 (and of Rh genes on chromosome 1). One of the 3 gene types (A, B, 0) determines blood group.- Individuals can be divided into 4 major blood group types on the basis of antigens (or agglutinogens) present on membrane of RBC. They are called agglutinogens as they are responsible for blood cell agglutination in transfusion reactions. Type A individuals have A antigen, type B have B, type AB have both A & B, and blood group type 0 have neither antigen.A and B antigens are complex oligosaccharides that differ in their terminal sugar moietyQ. H antigen is present in all blood types. H gene coding fucose transferase that adds a terminal fucose, forms H antigen. Individuals with type A express a second transferase that catalyzes placement of a terminal Nacetylgalactosamine on H antigen, whereas type B (individuals) express a second transferase that places a terminal galactose. Individuals with AB blood group have both and with 0 blood have neither transferases (i.e only H antigen persists in 0 group).Plasma antibodies against RBC agglutinogens are called agglutinins. Antigens very similar to A and B are very common in intestinal bacteria and possibly food to which new borns are exposed. These substances initiate the development of Anti A & Anti B antibodies. Therefore, infants rapidly develop antibodies against the antigens not present in their own cells. So type A blood group individual develop anti B, type B develop anti A, type 0 develop both, and type AB individual develop neither.Immediately after bih, the quantity of agglutinins in the plasma is almost zero. An infant begins to produce agglutinins, 2 to 8 months after bih and maximum titre is reached at 8 to 10 yrs. of age. Then it gradually declines throughout the remaining years of life.Anti A and anti B antibodies are isohaemagglutinins (iso antibodies) as they are produced by an individual against antigens produced by members of same species (isoantigens).Antibodies are usually 1gM type and cannot cross placenta. 0 type individual can produce IgG type antibodies.- The basic difference between ABO blood group system and other blood group systems (such as Rh, Kell, Duffy, MNSs blood groups), which makes the ABO group so impoant is that - preformed ABO antibodies are present in persons serum when his RBCs lack the corresponding antigen (ie. anti B antibody would be present in a person of type A and type 0 blood groups. These two blood groups do not have the 'B' antigen on the RBCs). This is not the case with other blood groups. Preformed antibodies are not present. They are formed only after an exposure to the antigen, for example Rh negative person do not have anti- Rh antibodies, these antibodies are formed only after an exposure to Rh positive blood (by a blood transfusion, i.e. this makes the first blood transfusion safe even if mismathced). These preformed antibodies, rapidly destroy the RBCs of any mismatched blood transfusion.- "In clinical transfusion practice, the ABO blood groups are the most impoant and can never be ignored in red cell transfusion, because individuals who genetically lack any antigen, have antibodies against the red cell types that they have not inherited. These antibodies can destroy red cells rapidly in circulation". But the question arises - why are these agglutinins (antibodies against AB antigen) produced in people who do not have the respective agglutinogens (A or B antigens) on their RBCs? The answer is that - Small amounts of group A and B antigens enter the body in food, in bacteria, and in other ways, and these substances initiate the development of the anti-A and anti-B agglutinins