the blood transfusion laboratory Flashcards
antigens
part of the surface of cells, all blood cells have antigens
antibodies
- protein molecules: immunoglobulins (Ig)
- usually IgM or IgG
- found in the plasma
- these are produced by the immune system in response to a foreign antigen
what are the 2 clinically most important blood groups?
ABO & Rh
what are the 3 factors that can stimulate antibody production?
- Blood transfusion (blood carrying antigens foreign to the patient)
- Pregnancy (Fetal antigens entering the maternal circulation)
- Environmental factors (naturally acquired e.g. anti-A)
describe antigen reactions in the body
destruction of the cell:
- directly when the cell breaks up in the blood stream (intravascular)
- indirectly when liver and spleen remove antibody coated cells (extravascular)
antigen reactions in the laboratory
reactions are normally seen as agglutination tests
what is agglutination?
is the clumping together of red cells into visible agglutinates by antigen-antibody reactions
* it results from the cross linking of antibodies and antigens
what can agglutination identify?
agglutination is antigen-antibody specific and so can identify the presence of the red cell antigen (blood grouping) and the presence of the antibody in the plasma (antibody screening)
ABO incompatibility
A or B cells being transfused into someone with the anti-A or anti-B antibody: can activate complement causing intravascular haemolysis
what happens in intravascular haemolysis
RBCs lyse in the circulation releasing haemoglobin into the plasma
Phenotype A
antigen: A
antibodies: anti-B
frequency: 43%
genotype: AA/AO
Phenotype B
antigen: B
antibodies: anti-A
frequency: 9%
genotype: BB/BO
Phenotype 0
antigen: none
antibodies: anti-A and anti-B
frequency: 45%
genotype: OO
Phenotype AB
antigen: A and B
antibodies: none
frequency: 3%
genotype: AB
Testing patient’s red cells with anti-A, anti-B and anti-D
- agglutination shows that a particular antigen is on the red cells
- no agglutination shows the antigen is absent
Testing patient’s plasma with A cells and B cells
- agglutination shows that a particular antibody is in the plasma or serum
- no agglutination shows the antibody is absent
Rh grouping system
Most important antigen is called D.
People with D antigen are D positive (85% of UK)
People who do not produce any D antigen are D negative (15%)
The other 4 main antigens are known as C, c, E and e
Rh typing
- must be tested in duplicate
- the patient/ donor will be classified as D positive or negative
what is the clinical significance of Rh?
- Transfusion
D antigen is very immunogenic and anti-D is easily stimulated - PREVENTION!
All Rh antibodies are capable of causing severe transfusion reaction- ANTIBODY DETECTION - Pregnancy
Rh antibodies are usually IgG and can cause haemolytic disease of the newborn.
Anti-D is still most common cause of severe HDN
how does haemolytic disease of the newborn arise?
- Rh+ father
- Rh- mother carrying her first Rh+ fetus. The Rh antigens can enter the mothers circulation during delivery
- Due to exposure to the fetal Rh antibodies, the mother will develop anti-Rh antibodies
- If the woman becomes pregnant with another Rh+ fetus, the anti-Rh antibodies cross the placenta and damage the RBCs
laboratory testing for HDN?
- blood group and antibody screen to identify risk of HDN
- may need prophylaxis
RAADP
An injection of anti-D will bind to and remove any fetal D positive red cells in the circulation
what is haemolytic transfusion reaction
serious complication that can occur after a bloodtransfusion. Thereactionoccurs when the red blood cells that were given during thetransfusionare destroyed by the person’s immune system.
how does antibody screening occur?
Patients serum is mixed with 3 selected screening cells, incubated for 15 minutes at 37oc and then centrifuged for 5 minutes.
Any clinically significant antibodies reacting at body temp should be detected and then identified using panel of known phenotyped red cells.
Specific antigen negative blood can then be provided for these patients to avoid stimulating an immune response.
what is zeta potential?
the positively charged ion cloud that surrounds RBCs
- IgGs are too small to overcome zeta potential
what does low ionic strength saline do?
it is negatively charged so neutralises the zeta potential
IgG is now able to span the gap between RBCs
Indirect Antibody Test
Used to detect IgG antibodies
LISS counteracts Zeta potential.
Results in agglutination
Immediate spin cross match
Antibody screen is negative
Checking donor red cells against patients plasma
ABO check
Incubate for 2 – 5 minutes (room temp), spin and read.
Full Indirect Antiglobulin test (IAT) cross-match
Antibody screen positive or patient has known antibody history.
Select antigen negative donor red cells and incubate with patient serum for 15 minutes at 37oC
Red cell transfusion
Concentrated red cells (packed cells) in a suspension of SAGM
Red cells oxygen carrying capacity
Symptomatic anaemia
Exchange transfusion
If significant bleeding anticipated, activate the major haemorrhage protocol
Fresh frozen plasma
FFP contains all clotting factors
Given for coagulopathy with associated bleeding
Requires clotting screens to monitor
Only has 24 hour life after thawing
(five days for major haemorrhage)
Platelets
Adult pool of platelets from 4 donors (suspended in plasma from 1 donor)
Platelets required to create clots to reduce bleeding
Some drugs given to reduce efficacy of platelets (anti-platelet agents) so patient history important
Cryoprecipitate
Contains Factor VIII, VWF and fibrinogen
2 units usually given at one time
Monitor fibrinogen levels by clotting screens
