Transfusion and transplantation

Question 1

Who are the recipients of blood donations

Answer 1

Recipients include leukaemia patients, sickle cell disease etc

Question 2

How many blood groups are there

Answer 2

There are 29 blood groups which carry two types of structure on their surface

Carbohydrates (AB0 – glycolipids)

Membrane protiens (Rh)

Question 3

What is the role of blood groups

Answer 3

Their role is to give the erythrocyte an overall negative charge – this aids repulsion in the capillaries

Question 4

What antigens do glycolipids carry

Answer 4

A, B or H antigens

Question 5

Why is it dangerous to receive a blood donation from someone with an unknown blood group

Answer 5

this is due to immune cells recognising these “foreign erythrocytes” and causes agglutination and therefore clotting

Question 6

What do erythrocytes lack compared to other cells

Answer 6

All nucleated cells in the body express either MHC-I or MHC-II – erythrocytes do not have this

Question 7

What do erythrocytes have in place of an MHC complex

Answer 7

They possess different cell surface molecules like glycolipids and Rhesus antigens

Both are immunogenic and will activate the adaptive immune response

Question 8

What can Rhesus antigens cause

Answer 8

blood group incompatibility

This is seen during transfusions as well as incompatible pregnancy

Question 9

Why are ABO and Rh screened for during a transfusion

Answer 9

They are the most immunogenic

Question 10

Roughly how many alleles does the A-group have

Answer 10

20

Question 11

What do the ABO antigens have in common with each other

Answer 11

Same carbohydrate core which contains the H antigen

Question 12

What are A and B antigens

Answer 12

Co-dominant

Question 13

What can ABO incompatibility cause

Answer 13

Symptoms within 24 hours or can be delayed several days

Back pain

Haemoglobinuria (red urine)

Chills (fever)

Jaundice (due to haemolysis of transfused blood)

Feeling of impending doom

Question 14

What are key aspects of ABO biochemistry

Answer 14

Fructosyltransferase (FUT1 gene) adds fucose subunits (glycosidic bonds)

Carbohydrates structures are widely expressed throughout the body, polymorphic due to genetic arms race es structure

Question 15

How does blood group O arise

Answer 15

Blood group O arise from a frame-shift mutation, causes a non-functional transferase

Question 16

Where does glycosylation occur

Answer 16

The Golgi complex

Question 17

Where can A, B and H antigens be found

Answer 17

A,B and H antigens are not just confined to erythrocyte surface – they can be secreted in saliva, gastric and seminal fluid

Question 18

What does carbohydrate secretion require

Answer 18

Carbohydrate secretion requires the Se (secretor) gene alpha-2-fucosyltransferase (FUT2) - is expressed in only 70-80% of the British population

Question 19

Where is FUT2 active and what does it create

Answer 19

FUT2 is active in epithelial cells (FUT1 catalyses the same reaction in erythrocytes)

FUT2 creates the “H antigen” where further glycosylation’s can be added

H antigen is central to carbohydrate secretion

Question 20

Characteristics of FUT1

Answer 20

Active in the mesoderm / myeloid cell lineages (erythrocytes) and catalyses the transfer of fucose alpha-1,2 glycosidic bonds to the terminal of a H-active substance (creates H antigen)

Question 21

Characteristics of FUT2

Answer 21

Active in endodermal cells (mucosa) and catalyses the identical reaction as FUT1

Produces Se[secretion] factor

Question 22

Characteristics of FUT3

Answer 22

Lewis (or Le) active substance and is active in endodermal cells – catalyses the transfer of fucose alpha-1,3 / 4 to N-acetylglucosamine

Question 23

What are the 2 types of Rhesus protein

Answer 23

RhD and RhCcEe

Question 24

Characteristics of Rhesus antigens

Answer 24

12 hydrophobic membrane spanning motifs and both rhesus antigens weigh roughly 30kDa

RhD and RhCcEe share close sequence homology (only differ by 30-35 residues) - 8 in extracellular and 24 intracellular/transmembrane domains

Question 25

How does Rh become expressed and how does it occur

Answer 25

Expression of Rh occurs in clusters at the cell surface (tetramer) - relies on the expression of RhAG

Question 26

What is the loci for the Rh gene and RhAG gene

Answer 26

The Rh gene is loci 1p34-36 RhAG loci is 6p11-21.1

Question 27

What are variations in Rh genes

Answer 27

In Caucasian Rh (-ve) the RhD gene is completely deleted – only the RhCcEe is expressed In black communities the RhD(psi) variant exists where the RhD gene is mutated and is non-functional The human Rh gene are highly polymorphic (>40 variations)

Question 28

How does the Rhesus antigen cause problems in future pregnancies

Answer 28

If the mother is negative or positive, the foetus can be the opposite This is fine during the pregnancy however foetal and maternal blood can mix during parturition Immunogenic, activates RhD reactive T and B cells 2nd pregnancy – risk of haemolytic disease of the new born Anti-D immunoglobulin neutralises foetal RhD in maternal blood Neonatal Fc receptor facilitates maternal IgG trans-migration Anti-D (IgG) attacks foetal blood --> haemolysis --> haem metabolites build up (bilirubin) and causes brain damage

Question 29

How can the mother's immune system be prevented from attacking the foetus due to Rhesus antigens

Answer 29

Prophylactic anti-D is given to some pregnant women

Question 30

How are platelets formed and why are they important

Answer 30

Thrombocytes arise from the common myeloid progenitor cells Under thrombopoietin (TPO) cytokine control Start as megakaryocyte Differentiate into platelets important for homeostasis

Question 31

What is Thrombocytopaenia and how is it caused

Answer 31

(shortage of platelets) Aplastic anaemia (gross haematopoietic defect in bone marrow) Cancer treatment (chemotherapy destroys haematopoiesis) Bone marrow transplant in leukaemia (irradiation)

Question 32

What are platelets central to

Answer 32

Platelets are central to haemostasis – collagen receptors detect tissue damage --> haemostatic plug formation (with other clotting factors)

Question 33

What is Plateletapheresis

Answer 33

whole blood is separated into constituents – platelets retained Donor recipient compatibility – HLA class-I antigens – platelets also express ABO antigens

Question 34

What are human platelet antigens

Answer 34

(HPAs) Are polymorphic Donor match is important – mis-matches can cause post-transfusion purpura (PTP) - an adverse reaction to blood transfusion HPA-1a, HPA-1b and HPA-5a are important antigens

Question 35

What are the degrees of immune reaction after tissue/organ transplantation

Answer 35

(graft rejection) Depends on the class of graft Autograft Self-tissue transferred from one part of the body to another (seen in burn patients) Isograft Involves monozygotic twins Allograft Tissues/organs are transferred between genetically different individuals (needs a good HLA match) Xenograft Tissue transferred between species

Question 36

How common are organ/tissue transplants

Answer 36

any organ / tissue transplants are routinely performed Majority of patients on organ transplant list >75% require a kidney

Question 37

What are the 2 types of rejection for allografts

Answer 37

First set rejection – rejection starts 7-14 days post-transplantation – full rejection occurs at 14 days Second set rejection – rejection starts 3-4 days post-translation with full rejection in 6 days In both types rejection is immune mediated

Question 38

What are important mediators in graft/organ rejection

Answer 38

Both CD4+ and CD8+ T cells are important mediators in the graft/organ rejection process – T helper cells play a central role

Question 39

How do antigens effect acceptance or rejection

Answer 39

Histocompatibility – tissues that have a good antigenic similarity, minimising rejection (MHC genes coded by HLA gene cluster) MHC is polymorphic – MHC mismatch is immunogenic ABO antigen also important in graft rejection – due to H-substances being expressed on endothelium and epithelium as well as erythrocytes (FUT2)

Question 40

How does tissue typing effect acceptance or rejection

Answer 40

Initial ABO typing (haemagglutination assay) MHC matching Lymphocyte toxicity assay – identifies MHC-I and MHC-II through incubation with various HLA-type antibodies (or DNA hybridisation or PCR of HLA alleles Cross matching – recipient antibodies against donor HLA

Question 41

What are hyperacute rejections

Answer 41

Occurs when there are pre-existing organ specific antibodies (rare) - patient has had repeated blood transfusions (H antigen mediated) Occurs very rapidly – before graft vascularisation (set 2)

Question 42

What are acute rejections

Answer 42

Cell mediated – 7-10 days post-transplantation (set 1) MHC mismatch T cells infiltrate can become activated and can cause tissue damage (macrophages are involved) and endotheliosis (graft may still be viable if immunosuppressive therapy is used)

Question 43

What is the chronic phase of organ rejection

Answer 43

Happens months-years after Chronic immune activity which culminates in damaged vessel wall (growth and repair) leading to vessel occlusion – blood can no longer supply the organ causing the organ to die

Question 44

How does immunosuppression help

Answer 44

Histocompatibility mismatch means, overtime, all grafts and organs will be rejected over time This can be improved by using immunosuppressive drugs Imuran Is an azathioprine Inhibits mitosis and therefore T cell proliferation Cyclosporine Targets calineurin – a downstream phosphatase in TCR signalling pathway

Question 45

How are bone marrow transplants used

Answer 45

Needed for treating blood cancers like leukaemias and lymphomas Treats haemolytic disease such as sickle cell and thalassemia (affects production of haemoglobin) Aplastic anaemia is also treated using this

Question 46

How does AML arise

Answer 46

Genetic mutation occurs in myeloid progenitor cell Proliferation of undifferentiated myeloblast clones (blood cancer cell ; blasts) Overcrowd bone marrow and affects normal haematopoiesis – prevents platelets, erythrocytes and neutrophils from being produced Leads to bone marrow failure and ultimately death

Question 47

How does a bone marrow transplant help treat AML

Answer 47

Is challenging to treat – standard chemotherapy dosage is reduced (due to patients being over 60 typically) BMT offers greatest chance of prolonging survival (patient irradiated before donor bone marrow transfusion) Studies into childhood leukaemia show importance of HLA mismatch in bone marrow graft survival Close MHC-II matches are documented to reduce the risk of failure – DQ and Dr alleles are particularly important MHC-I A, B and C can also contribute

Question 48

How can graft vs host disease (GVHD) affect BMTs

Answer 48

HLA mismatch BMT carries the risk of GVHD Alloreactive T cells from the donor bone marrow mount an immune response towards recipient tissues Can be ameliorated through T cell depletion using antibodies generated toward certain T cell populations GVHD risk is low for solid organs Host vs graft (graft rejection) is high though