14. The Blood Transfusion Lab Flashcards
What is the difference between antigens and antibodies?
Antigens are part of the surface of cells
- All blood cells have antigens
Antibodies are protein molecules ~ immunoglobulins (Ig)
- Usually of the immunoglobulin classes: IgG and IgM
- Found in the plasma
- Produced by the immune system following exposure to a foreign antigen
Reactions to blood usually occurs when the antibody in the plasma reacts with an antigen on the cells
Describe the known blood groups.
There are 26 known blood group systems
ABO and Rh are clinically most important
Antigens in transfused blood can stimulate a patient to produce an antibody but only if the patient lacks the antigen themselves
The frequency of antibody production is very low but increases the more transfusions that are given
What can stimulate antibody production?
- BLOOD TRANSFUSION
i. e. blood carrying antigens foreign to the patient - PREGNANCY
Fetal antigen entering maternal circulation during pregnancy or at birth - ENVIRONMENTAL FACTORS
(i. e. naturally acquired e.g. anti-A and anti-B)
What are the different antibody-antigen reactions?
IN VIVO (IN THE BODY) ~ leads to the destruction of the cell either:
- directly when the cell breaks up in the blood stream (intravascular)
- indirectly when liver and spleen remove antibody coated cells (extravascular)
IN VITRO (IN THE LABORATORY) - reactions are normally seen as agglutination tests
What is agglutination?
- Agglutination is the clumping together of red cells into visible agglutinates by antigen-antibody reactions
- Agglutination results from antibody cross-linking with the antigens
What can agglutination help us identify?
As the antigen-antibody reaction is specific, agglutination can identify:-
- The presence of a red cell antigen. i.e. blood grouping
- The presence of an antibody in the plasma i.e. antibody screening/identification
What is the clinical significance of the ABO grouping system?
- A and B antigens very common (55% UK)
- Anti-A, anti-B or anti-A,B antibodies very common (97% UK)
- High risk of A or B cells being transfused into someone with the antibody in a random situation
- ABO antibodies can activate complement causing INTRAVASCULAR HAEMOLYSIS
How would you identify a patient’s blood group via lab testing?
The patient’s red cells and plasma are both tested
1) Test 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
2) Test 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
*From these results, using the blood group table, we can interpret the data and figure out the patient’s blood group
Describe the Rh grouping system
- 50+ antigens:
- Most important antigen is called D.
- People with D antigen are RhD positive (85% of UK)
- People who do not produce any D antigen are RhD negative (15%)
- The other 4 main Rh antigens are known as C, c, E and e
- Most important after ABO
- Must be tested in duplicate (or tested each time and compared to historical result)
- Patient / Donor classified as RhD pos or RhD neg
What is the clinical significance of Rh grouping?
*Rh antibodies are clinically significant ~ Second only to ABO
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
Describe an example of how HDN works
- Haemolytic disease of the newborn
1) There is a Rh+ father and a Rh- mother.
2) The Rh- mother is carrying her Rh+ foetus. Rh antigens from the developing foetus can enter the mother’s blood during delivery.
3) In response to the foetal Rh antigens, the mother will produce anti-Rh antibodies.
4) It he woman become pregnant with another Rh+ foetus, her anti-Rh antibodies will cross the placenta and damage the foetal red blood cells.
How would you use laboratory screening to prevent HDN consequences?
- Blood group and antibody screen at antenatal booking to identify pregnancies at risk of HDN ~ RhD negative women who may need anti-D prophylaxis
- Blood group and antibody screen at 28 weeks
- Atypical antibodies are quantified periodically to assess their potential effect on the fetus
What do we do when there is an RhD- woman who is pregnant?
- An injection of anti-D prophylaxis or RAADP will bind to and remove any fetal RhD positive red cells in the circulation
- 1500 iu of anti-D is given routinely at 28 weeks and a smaller dose (usually 500 iu) after delivery if baby RhD+
- In some hospitals 2 smaller (500 iu) doses are given at 28 and 34 weeks instead of the 1 larger dose
- Anti-D is also given after any event that may cause a feto-maternal haemorrhage (bleed between mum and fetus) such as:
- Abdominal trauma
- Intrauterine death
- Spontaneous or therapeutic abortion
Why do we perform antibody screening?
- There are other clinically significant antibodies that can cause a haemolytic transfusion reaction.
- It is important that we screen for these antibodies so that If detected, antigen negative blood can be provided to avoid causing a immune reaction.
How do we perform antibody screening?
- 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.
