Blood transfusion testing Flashcards
define antigens
any substance, which in appropriate biological circumstances, can stimulate an immune response
define antibodies
protein molecules (immunoglobulins) which are produced in response to the introduction of a foreign antigen
what are the two circumstances when blood group antibodies are produced
in response to environmental antigen
in response to a red cell antigen
what are the two main responses of antigen - antibody reactions
aggultination
haemolysis
what are the two stages of agglutination
Sensitisation
cross linkage
what factors affect sensitisation
chemical bonds antigen:antibody ratio temperature pH incubation time ionic strength
what is cross linkage
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.
what factors affect cross linakge
size / physical properties of antibody molecule
concentration of antigen sites on the cell
distance between cells
what are the two main antibodies for blood grouping
IgG
IgM
what is the differences between IgG and IgM when it comes to testing
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
why cant IgG antibodies bridge the gap between red cells
due to the red cells Zeta potential.
what is the difference between IgG and IgM reagents
IgG are indirect agglutinins
IgM Direct agglutinins
what are the advantages of saline testing
simple
best way to detect IgM antibodies
what are the disadvantages of saline testing
will not detect IgG antibodies
may pick up clinically insignificant IgM antibodies at room temp
what are the two Saline types
Normal Ionic Strength Saline (NISS)
Low Ionic Strength Saline (LISS)
what are antiglobulin test
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
what are the two types of aniglobulin tests
Direct antiglobulin test (DAT)
Indirect antiglobulin test (IAT)
what are the differences between DAT and IAT
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
what are enzyme tests
certain antigens become more exposed whilst others are destroyed (enzyme proteolysis)
what is the mechanism of action for enzyme testing
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
Name four proteolytic enzymes
Papain (papaya)
Ficin (figs)
Bromelin (pineapples)
Trypsin (lining of a hogs stomach)
what is elution and what happens
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
what are the applications of Elution
investigate positive DAT to identify implicated antibodyin:
foetal red cells in HDN case
post transfusion reaction
autoantibody
concentration or purification of antibodies
what are possible sensitising events of Anti D
anti D immunoglobulin given after potentially sensitising events including bleeding adnominal trauma miscarriage delivery of RhD positive infant
