Lecture 17 Blood Group Serology introduction Flashcards
What are Blood Group Antigens?
- Human blood cells contain on their surface a series of glycoproteins and glycolipid__s, which constitute blood group antigens.
- Some (e.g. ABO) may also be present more widely on endothelial surfaces.
- Development of these antigens is genetically determined (appear early in fetal life and remain unchanged until death).
- Inheritance of all of blood group systems is determined by autosomal genes (co-dominant), with exception of Xg system which is determined by genes on X chromosome.
- So you inherit one gene from each parent and you normally express the product of both genes on surface of the cells.
- The only that is not is the Xg system.
- Theories on the biological role of human blood group systems (antigens) have been postulated, but despite much discussion and speculation their purpose remains unclear.
- Possible functions are recognition of self and non-self, the maintenance of cellular integrity, involvement in cell maturation and the susceptibility or resistance to human disease.
Describe the Genetic control of blood groups
Genetic Control of Blood Groups
- Protein Determinants
- Gene codes for antigenic determinant itself,
- e.g. Rh, Kell, Duffy, Kidd systems
- Glycolipid Determinants
- Gene codes for production of enzymes that add or remove carbohydrate or lipids,
- __e.g. ABO, Lewis group systems
Functional Aspects of Blood Group Antigens
- Duffy Blood Group System And Malaria
Duffy antigen acts as the entry point to the red cell for the malarial parasite.
- In caucasian populations, Fya– Fyb– phenotype is rare
- In black African populations, up to 40% are Fya– Fyb-, this represents impact of natural selection for malarial resistance
- The McLeod Phenotype
Kx null phenotype associated with chronic granulomatous disease and acanthocytosis
Describe 2 Functional Aspects of Blood Group Antigens
In whole, we don’t know why the antigens are there, but we know the functions of these 2.
Functional Aspects of Blood Group Antigens
- Duffy Blood Group System And Malaria
Duffy antigen acts as the entry point to the red cell for the malarial parasite.
- In caucasian populations, Fya– Fyb– phenotype is rare
- In black African populations, up to 40% are Fya– Fyb-, this represents impact of natural selection for malarial resistance
- The McLeod Phenotype
Kx null phenotype associated with chronic granulomatous disease and acanthocytosis
Describe Blood Group Antibodies
- Blood group systems are of importance in clinical medicine because of their ability to stimulate antibody formation (implications for transfusion, transplantation and also several specific immune diseases in newborn)
- Blood group antibodies recognize foreign antigens.
- Antibodies may be IgM, IgG, IgA.
- It can be naturally occurring or immune stimulated.
- Naturally occuring antibodies will be produced with no exposure to foreign red cells
- Immune stimulated antibodies will be produced in response to foreign red cells, by transfusion or pregnancy.
- It can be naturally occurring or immune stimulated.
Describe the two types of RED CELL antibodies
Naturally Occurring Red Cell Antibodies
These antibodies develop in the absence of exposure to corresponding red cell antigen. ABO and Lewis antigens fall in this category.
These antibodies are not present at birth but develop during first year of life
- Most likely stimulated by cross reacting antigens derived from bacteria (develop as an immune response to substances found in environment with similar antigenic determinants)
- Usually relate to lipid antigens.
Naturally occurring antibodies have significant IgM component, although IgG may also be present.
Immune Stimulated Red Cell Antibodies
These antibodies develop only following exposure to specific foreign red cells /corresponding antigens.
It may be produced following:
- Transfusion of red cells containing the antigen
- Transplacental haemorrhage (pregnancy) where fetal red cells enter the maternal circulation
- Injection of small numbers of red cells in contaminated syringes (e.g. IVDU)
Immune antibodies are normally IgG in nature.
- Often they are active at 37 degrees Celsius. These antibodies may bind early complement factors but do not normally result in activation of the final haemolytic part of complement cascade.
- Red cell destruction is thus normally extravascular occurring in the spleen and liver.
Immune red cell antibodies are normally ____ in nature.
Immune antibodies are normally IgG in nature.
What are the 4 main differences between the characteristics of Red Cell Antibodies
Naturally occuring
- Usually glycolipids
- IgM +/- IgG
- Naturally occuring antibodies have the ability to activate the complement system (DANGEROUS)
- Site of red cell destruction is intravascular
Immune stimulated
- Usually glycoprotein
- IgG
- No or early phase only (to C3)
- The setting of red cell destruction is extravascular
Describe the features of the ABO antigens
Basic Features
- ABO blood group system was first identified by Landsteiner in 1901. It was first blood group system to be defined and clinically most important.
- ABO antigens are widely distributed, including red cells, platelets, granulocytes, epithelial cells. Soluble forms of ABO antigen are also present in body fluids.
- Phenotype is determined by a series of glycosyltransferase enzymes. These are responsible for addition of carbohydrate molecules onto the basic membrane structure.
- H antigen is necessary for ABO phenotype to be expressed.
- The only difference between O, A and B are the terminal sugars
- A = N Acetyl Galactosamine
- B = D Galactose
- O = Nil
What are the terminal sugars on each of the ABO antigens?
The only difference between O, A and B are the terminal sugars
A = N Acetyl Galactosamine
B = D Galactose
O = Nil
What are the different types of Blood groups and for each describe the
1) Genotype
2) Phenotype
3) Antibodies present
(note that if you have the genotype, you will lack the antibodies and vice versa)
It is the presence of Antibodies that renders them so important.
Describe the Clinical Relevance of the ABO system
Clinical Relevance Of ABO Bood Group
Naturally occurring antibodies are found regularly in persons of blood group O, A and B (not AB). They appear in the serum of infants between 3-6 months age.
Transfusion of ABO incompatible red cells result in complement activation, which will lead to:
- Intravascular haemolysis
- Disseminated intravascular coagulation (DIC)
- Renal failure
10% cases are fatal. This is the most frequent cause of death following incompatible blood transfusion and normally results from human error.
Normally in clinical practice, when an individual requires a transfusion they should be transfused with blood of the same ABO group. In some settings, this is not possible. In such circumstance, ABO compatible red cells should be transfused, i.e. transfused red cells should be compatible with plasma of the recipient (i.e. lack the antigen to corresponding ABO antibody present in plasma).
- Blood group O is universal donor group (safely transfused to any ABO group). It is used for emergency transfusions (insufficient time for blood group of patient to be determined).
- Blood group AB is universal recipient. Such individuals can safely receive red cells of any ABO group.
Transfusion of ABO incompatible red cells result in complement activation, which will lead to…
- Intravascular haemolysis
- Disseminated intravascular coagulation (DIC)
- Renal failure
Blood group O is ________________
Blood group AB is ___________________
Blood group O is universal donor group (safely transfused to any ABO group). It is used for emergency transfusions (insufficient time for blood group of patient to be determined).
Blood group AB is universal recipient. Such individuals can safely receive red cells of any ABO group.
Describe the Rh Blood Group System
Introduction
Rh system is the second most important blood group system, discovered in 1939.
- It is restricted to red cell and is not expressed on other blood cells or tissues.
- It is protein determinant. It has a highly immunogenic nature, particularly Rh(D) antigen.
Rh(D) Status
All individuals can be either Rh(D) positive or Rh(D) negative.
- Gene controlling Rh(D) negative group is an amorph (i.e. it does not result in production of any antigen).
- Essentially, Rh(D) negative group is defined simply by absence of Rh(D) glycoprotein.
What is the second most important blood group system?
Rh Blood Group System
What are the genotypes and phenotypes of the Rh Blood Group System?
DD, Dd = positive
dd = negative
Describe the Laboratory Techniques to check the blood types
Routine testing used in blood group serology to identify antigens and antibodies utilises agglutination technique.
Essentially, red cells are incubated with serum. If serum _contains an antibody t_argeted against an antigen present on the surface of the red cell, then antibody will bind to antigen.
- Antigen testing usually uses commercially sourced monoclonal reagents.
- These have high sensitivity and specificity. They are IgM in nature. They produce direct agglutination.
Red cells are negatively charged and the charge (zeta potential) separates red cells in an anticoagulated sample.
- IgM antibody is a large pentamer. Antibody binding sites of IgM molecule are sufficiently far apart than zeta potential to enable cross-linking of red cells. Therefore, it will lead to direct agglutination of sensitised red cells.
-
IgG antibody is a dimer. Distance between the two antibody binding sites is less than effect of zeta potential, unable to product cross-linking.
- Therefore, IgG antibody will sensitise cells, but a potentiator is need to produce agglutination, and
- most frequently we use anti-human globulin (AHG),
- Often produced by injecting rabbits with human blood- then they develop antibodies to that blood antigen.
- others include enzymes, albumen.
- most frequently we use anti-human globulin (AHG),
- Therefore, IgG antibody will sensitise cells, but a potentiator is need to produce agglutination, and
AHG comprises a mixture of antibodies to human IgG and C3 complement. These may be monoclonal in nature or produced in animal models.
The anti-human IgG allows bridging of red cells sensitised with IgG antibodies, this reaction resulting i_n visible agglutination._
AHG reaction is widely used in transfusion medicine, both to detect red cell antibodies and also in pretransfusion testing.
Describe the pathophysiology of Haemolytic Disease of Fetus and Newborns
- Small number of fetal blood can cross into the maternal circulation (feto-maternal haemorrhage)
- The mother can develop an antibody to the antigens on the fetus.
- Normally little happen in the first pregnancy because the antibody production level isn’t sufficiently high or sustained to cause harm.
- If the mother has a second pregnancy, the motehr will already have antibodies against the antigen that was present on the first child.
- The second child has a 67% chance of having the D antigen. If the D antigen is present, the antibodies in the maternal circulation will cross the placenta into the fetal circualtion and destroy
- In its most severe form (rare), this results in severe fetal anaemia and ultimately death.
- If they are born alive, they will still have antibodies, and there will be continuous destruction of the Red Blood cells
- Can result in a range of neurological dieases.
