Blood Groups Flashcards
Define an antigen
a molecule with the capacity to be recognised by the immune system
What is substance H ?
- carbohydrate
- ubiquitously expressed
- visible
- modified
Describe the ABO gene
- maps to long arm (q) of chromosome 9
- encodes a protein that can possess enzyme activity, glycosyltransferase
- 3 alleles –> A,B, & O
Describe allele A
- encodes an active enzyme which catalyses the transfer of N-acetylgalactosamine to substance H
Describe allele B
- encodes an active enzyme which catalyses the transfer of galactose to substance H
Describe Allele O
- encodes a protein with no enzymatic activity
- frame-shift mutation in gene
- substance H remains in native conformation
Which alleles are co-dominant?
A & B
Which allele is recessive ?
O
What are the genotypes for the phenotype for blood group A ?
- AA
- AO
What are the genotypes for the phenotype for blood group B ?
- BB
- BO
What are the genotypes for the phenotype for blood group AB ?
- AB
What are the genotypes for the phenotype for blood group O ?
- OO
Briefly describe the Indirect Antiglobulin Test (IAT)
- detection of anti-A or anti-B in a serum sample
- test serum is applied to RBCs of known phenotype; A,B, AB,O
- Ab’s bind
- RBCs washed
- anti-human globulin added
- agglutination of RBCs occurs if the serum contained antibodies against the red cell phenotypes
What can cause Anaemia ?
- blood loss
- RBC destruction
- Lack of RBC production
What is an Immediate haemolytic transfusion reaction?
- usually ABO associated, complement activating
- massive intravascular haemolysis, immediate & life threatening
Describe the Rh Locus
- maps to short arm (p) of chromosome 1
- 2 related genes –> D & CE
- Rh status is compound –> D & CE
Describe Rh D
- most clinically significant Rh antigen
- gene is either present or absent
- 2 phenotypes –> RhD+ or RhD-
Describe the Rh CE gene
- gene is always present
- expression pattern of gene generates 4 phenotypes
- RNA splicing
Describe Rh Antibodies
- not naturally occurring
- immune response against Rh antigen is required to generate antibodies
- presence of Rh antibody indicates a previous “immunisation” against Rh
- anti-RhD also known as rhesus factor
Describe Rh D in pregnancy
- Rh D status determined in all expectant mothers
- RhD expectant mothers are offered anti-D therapy at 28 & 34 weeks gestation
- Anti-D therapy coat, and triggers destruction of any foetal RBCs that may have crossed into the maternal circulation
- reduces risk of mothers immune system detecting RhD+ foetal cells & mounting an immune response
Describe Rh Haemolytic disease of the newborn
- mothers immune system generates anti-D
- Anti-D IgG crosses the placental
- Anti-D IgG binds in foetal circualtion
- triggers destruction of foetal RBCs
- IgG are not complement activating
- extravascular haemolysis in unborn child/neonate
What lab investigations are there for Rh HDN ?
- Coombes test - neonate sample or cord blood
- positive if foetal cell has been coated with anti-D in vivo
