blood transfusion Flashcards
what does the clinical importance of a blood group system depend on?
the capacity of antibodies against the specific RBC antigens to cause haemolysis of the RBCs
What are the two classes of diseases/conditions that clinically significant antibodies against RBC antigens can cause?
HTRs and HDFN
What is HTR
haemolytic transfusion reactions
incompatible red cells are transfused - so the transfused RBCs have the antigen corresponding to antibodies in the patient’s plasma
what is HDFN
haemolytic disease of the foetus and newborn
the foetus has a different RBC antigen to the mother, and the mother has produced an antibody to that RBC antigen which has crossed the placenta.
What are the naturally occurring ABO antibodies?
anti-A and anti-B
antigens may be encountered in foods or micro-organisms
what class are ABO antibodies mostly
IgM antibodies
What type of haemolysis- caused reaction do the IgM antibodies cause?
acute HTRs through activation of the complement system
results in massive intravascular haemolysis
when are acquired antibodies formed?
as a result of active immunisation to non-self RBC antigens following exposure to RBCs eg from another person
are all alloantibodies clinically significant?
no
What class are acquired alloantibodies usually?
IgG
What can acquired alloantibodies cause?
mainly extravascular haemolysis resulting in delayed HTRs
IgG antibodies can cross the placenta and cause HDFN
what does the A gene code for?
an enzyme which adds N-acetyl galactosamine (GalNac) to the common H-antigen, making the A antigen
what does the B gene code for?
an enzyme which adds galactose (Gal) to common H antigen to form the B antigen
what does the O gene code for?
an enzyme which adds galactose (Gal) to common inactive enzyme
Why do IgG antibodies not usually cause HDFN despite being able to cross the placenta?
- fetal red cells have poorly developed ABO antigens, they are unable to support the binding of the IgG antibodies
- ABO antigens are found on numerous other cells- any IgG ABO antibodies which have crossed the placenta will bind to the other cells
- maternal IgG anti-A or anti-B antibodies present at birth in a baby’s plasma will disappear within a few months when the baby develops their own IgM antibodies
universal red cell donor?
group O (no A or B antigens so no acute HTR despite patient’s antibodies)
universal plasma donor?
group AB (no antibodies so no HTR)
inheritance pattern of D antigen
autosomal dominant
How long after exposure does I take for anti-D antibodies to become apparent?
2 to 5 months
what can anti-D antibodies cause?
delayed HTRs - extravascular, if RhD positive red cells are transfused- this results in anaemia, high bilirubin and therefore jaundice
HDFN (important) if a mother has IgG anti-D antibodies and the baby is RhD positive, the IgG antibodies can cross to the placenta which can lead to intra-uterine death or brain damage from high levels of bilirubin
how do you prevent the formation of anti-D antibodies?
ensure RhD negative patients receive RhD negative red cells and platelet transfusions
in PREGANT WOMEN:
give them anti-D immunoglobulin! destroys any RhD positive foetal RBCs in the maternal circulation before the mother has time to create an immune response.
Types of Pre-transfusion compatibility testing
ABO, RhD and antibody screen
collectively known as “group and screen”
what’s the forward group in regards to ABO resting
when the patient’s RBCs are tested against reagent anti-ABO antibodies
interaction results in agglutination which can be seen
so agglutination w the anti reagent for a specific blood group means that it’s that blood group present
what’s the reverse group in regards to ABO testing?
when the patient’s serum is mixed with reagent A and B RBCs
interaction results in agglutination
