Blood transfusion testing

Question 1

define antigens

Answer 1

any substance, which in appropriate biological circumstances, can stimulate an immune response

Question 2

define antibodies

Answer 2

protein molecules (immunoglobulins) which are produced in response to the introduction of a foreign antigen

Question 3

what are the two circumstances when blood group antibodies are produced

Answer 3

in response to environmental antigen

in response to a red cell antigen

Question 4

what are the two main responses of antigen - antibody reactions

Answer 4

aggultination

haemolysis

Question 5

what are the two stages of agglutination

Answer 5

Sensitisation

cross linkage

Question 6

what factors affect sensitisation

Answer 6

chemical bonds
antigen:antibody ratio
temperature
pH
incubation time
ionic strength

Question 7

what is cross linkage

Answer 7

antibodies attach to the antigens on the surfaces. forms a lattice which can be seen in testing as cells settle by gravity and can be centrifuged.

Question 8

what factors affect cross linakge

Answer 8

size / physical properties of antibody molecule

concentration of antigen sites on the cell

distance between cells

Question 9

what are the two main antibodies for blood grouping

Answer 9

IgG

IgM

Question 10

what is the differences between IgG and IgM when it comes to testing

Answer 10

IgM antibodies are large multivalent molecules which easily causes direct agglutination

IgG molecules are smaller monomers that are generally unable to bridge the gap between red cells. Causes senstisation without lattice formation

Question 11

why cant IgG antibodies bridge the gap between red cells

Answer 11

due to the red cells Zeta potential.

Question 12

what is the difference between IgG and IgM reagents

Answer 12

IgG are indirect agglutinins

IgM Direct agglutinins

Question 13

what are the advantages of saline testing

Answer 13

simple

best way to detect IgM antibodies

Question 14

what are the disadvantages of saline testing

Answer 14

will not detect IgG antibodies

may pick up clinically insignificant IgM antibodies at room temp

Question 15

what are the two Saline types

Answer 15

Normal Ionic Strength Saline (NISS)

Low Ionic Strength Saline (LISS)

Question 16

what are antiglobulin test

Answer 16

uses anti-human globulin reagents. (antibodies to human immunoglobulin)
allows sensitised cells to be agglutinated
reaction of IgG antibodies with their antigen can be detected

Question 17

what are the two types of aniglobulin tests

Answer 17

Direct antiglobulin test (DAT)

Indirect antiglobulin test (IAT)

Question 18

what are the differences between DAT and IAT

Answer 18

DAT
demonstrates in vivo coating of red cells with antibody or complement
washed red cells are tested directly with AHG reagent

IAT
plasma incubated with red cells
in vitro coating red cells with antibody or compliment
cells washed to remove unbound globulins
agglutination when AHG added indicates antibody bound to specific antigen on red cells

Question 19

what are enzyme tests

Answer 19

certain antigens become more exposed whilst others are destroyed (enzyme proteolysis)

Question 20

what is the mechanism of action for enzyme testing

Answer 20

proteolytic enzymes reduce the red cell surface charge by cleaving polypeptides containing the negatively charged Sialic acid.

reduction of net charge enhances agglutination

red cells pre-teated with proteolytic enzymes show enhanced agglutination by IgG molecules.

Enzymes destroy certain red cell antigens. eg. M,N,S,FYa and FYb

Question 21

Name four proteolytic enzymes

Answer 21

Papain (papaya)
Ficin (figs)
Bromelin (pineapples)
Trypsin (lining of a hogs stomach)

Question 22

what is elution and what happens

Answer 22

Frees antibody from sensitised red cells

bound antibodies are released by:
changing thermodynamics of Ag-Ab reactions
reversing forces of attraction that bind Ag and Ab together
distrubs structure of Ag-Ab binding site

recover bound antibody in stable state

Question 23

what are the applications of Elution

Answer 23

investigate positive DAT to identify implicated antibodyin:
foetal red cells in HDN case
post transfusion reaction
autoantibody

concentration or purification of antibodies

Question 24

what are possible sensitising events of Anti D

Answer 24

anti D immunoglobulin given after potentially sensitising events including 
bleeding
adnominal trauma
miscarriage
delivery of RhD positive infant