blood group systems.pptx

Haematology

1. Blood group systems

2. Red cell antigens that are produced by alleles ( alternative forms of a specified gene) at a single gene locus or very closely linked loci constitute a blood group system. Blood group genes are inherited in a Mendelian manner and are mostly located on autosomes . Most of the blood group genes are expressed in a codominant manner ( i.e. the two allelic forms are expressed equally if inherited in a heterozygous state ).

3. In blood transfusion practice, most important blood group systems are ABO and Rh . This is because A, B, and Rh D antigens are the most immunogenic (i.e. capable of eliciting a strong antibody response on stimulation) and their alloantibodies can cause destruction of transfused red cells or induce haemolytic disease of newborn (HDN ). ABO antigens are also important in organ transplantation .

4. ‰‰‰ ABO SYSTEM four main types of blood groups–A, B, AB, and O two major subgroups of A: A1 (80%) and A2 (20%). anti-A1 antibodies are weak and are of little clinical signifiance in routine practice.

5. Antigens of the ABO System: ABO antigens are carbohydrate structures on glycoproteins and glycolipids. ABO antigens are poorly expressed at birth, increase gradually in strength and become fully expressed around 1 year of age. In older age, they become slightly weak.

6. (HH, Hh )

7. Some persons do not inherit the H gene (genotype hh ) and thus cannot synthesise H substance. Such persons may inherit the A or B gene but cannot express it, as they are unable to produce the H substance. Such individuals are said to have Bombay phenotype or Bombay blood group (Oh).

8. Antibodies of the ABO System naturally occurring antibodies because they arise without immune stimulation (i.e. transfusion or pregnancy) by relevant blood group antigens . not detectable in the blood of newborn infants due to their underdeveloped immune system and appear around 3 to 6 months of life. produced in response to A- and B-like antigens of bacteria, which are present in the intestine and certain foods.

9. ‰‰‰ THE Rh SYSTEM high immunogenicity of Rh D antigen, which readily induces formation of anti-D antibodies in Rh D-negative individuals. Anti-D antibodies can cause haemolytic transfusion reaction or, in pregnant women, Rh haemolytic disease of newborn.

10. Antigens of the Rh System important antigens of the Rh system are C, D, E, c, and e. D antigen is the most immunogenic. The presence of D in either homozygous (D/D) or heterozygous (D/d) state makes that individual Rh positive , while Rh negative persons are homozygous for d (d/d).

11. In Rh negative persons, deletions, point mutations, or partial mutations of D gene have been found . Rh antigens are expressed only on red cells and not on any other tissues. They are also not secreted in body fluids . In contrast to ABO antigens , Rh antigens are fully expressed on red cells before birth and also on red cells of early foetuses .

12. weak D antigen: react weakly with anti-D reagent There is a quantitative reduction in the number of D antigen sites on such red cells . Du recipients do not make anti-D antibodies following stimulation by D antigen (e.g. following D+ve blood transfusion)

13. Du donors should be considered as Rh positive and their blood should not be transfused to Rh-negative donors

14. partial D antigen parts of D antigen are missing . Variants of partial D antigen exist . Individuals with DVI variant are able to produce anti-D antibody against the missing part of the antigen if exposed to D+ve antigen . Such recipients should be considered as Rh negative, while donors should be regarded as Rh positive.

15. in practice, individuals with partial D antigen are typed as D negative and are identifid only after they have produced anti-D antibodies.

16. Rh Antibodies Rh antibodies are of immune type, i.e. they are the result of immunisation by blood transfusion or pregnancy. Most of these antibodies are of IgG class .

17. In practice, Rh antibodies can cause haemolytic transfusion reaction or haemolytic disease of newborn. Since Rh antibodies do not activate complement, haemolysis is extravascular and predominantly occurs in spleen. Due to high immunogenicity of D antigen, Rh-negative persons (especially women of child bearing age) should be transfused only with Rh-negative blood