This document summarizes important blood group systems, focusing on ABO and Rh systems. It describes how blood group antigens are inherited and expressed, and the clinical significance of various blood group antibodies. The ABO system has four main blood types (A, B, AB, O) based on carbohydrate antigens. The Rh system's most important antigen is D, which readily induces anti-D antibodies in Rh-negative individuals, requiring careful blood typing to avoid transfusion reactions or hemolytic disease of the newborn.

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).

4. • 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.

5. ‰
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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.

7. 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.

8. (HH, Hh)

9. • 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).

10. 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.

11. ‰
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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.

12. 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).

13. • 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.

15. 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)

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

17. 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.

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

19. 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.

20. • 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