HIS26 Introduction To Immunohaematology And Hospital Blood Banking Flashcards
Blood groups
Expression of Ag on RBC membrane
- proteins / carbohydrates
- many blood groups (ABO, Rh, Duffy, Kidd etc.)
- ABO, Rh most important (∵ most immunogenic)
Significance of different blood groups depends on:
- Frequency of Ag in population
- Effect of corresponding Ab on RBC
Significance of Blood group Ab
- Class/subclass of Ig
- Titre of Ab
- Thermal amplitude
—> Warm Ab: IgG (active at body temp)
—> Cold Ab: IgM (1-4oC) - Potency of Ab:
- Ability to fix complement
- Ability to cross placenta (haemolytic disease of fetus)
- Some Ab are naturally occurring e.g. Anti-A, Anti-B (***Isohaemagglutinins)
- Most Ab are acquired (after exposure to corresponding Ag)
***ABO blood group
Most important of all blood group systems
Naturally occurring Ab:
- Anti-A, Anti-B
- **IgM (Cold Ab but active at body temp)
- can fix complement —> **Haemolytic
- ***cannot cross placenta
AB Antigen on RBC:
- ***Oligosaccharides
some Anti-A / Anti-B can be immune in origin (i.e. IgG) —> can cross placenta (but haemolytic disease caused usually mild / self-limiting)
***ABO gene
- located on Chromosome 9
- encode enzyme with **Glycosyltransferase activity which **modify oligosaccharides on surface membrane glycoprotein
- default oligosaccharide structure - ***H-antigen (H gene on chromosome 19) —> precursor of A, B antigen
“A” allele: encode an enzyme adding ***N-acetyl galactosamine to H-antigen —> convert it to A antigen
“B” allele: encode an enzyme adding ***galactose to H-antigen —> convert it to B antigen
“O” allele: encode protein with ***loss of transferase activity (modify nothing on RBC) —> contain no Ag on RBC
ABO genotypes and phenotypes
Inheritance: ***Autosomal dominant
A, B: Co-dominant
O: Recessive
AA, AO —> A blood group
BB, BO —> B blood group
AB —> AB blood group
OO —> O blood group
Groupings of ABO blood group
- Cell grouping: presence/absence of A/B antigens on RBC
- Plasma / serum grouping: presence/absence of Anti-A/Anti-B in serum
If a particular Ag is present, there is no corresponding Ab
***Significance of ABO system
- Acute haemolytic transfusion reaction:
- Hyperacute ejection
- due to ABO incompatibility (ONLY reason)
- incompatible RBC lysed by ***Complements activated by naturally occurring IgM ABO Ab reactive at body temp
- fatal if unrecognised / untreated - ABO haemolytic disease of newborn (HDN):
- due to immune ABO Ab (IgG) by mother crossing placenta to destroy RBC of fetus
- more common in group O mothers (have Anti-A + Anti-B) with non-O fetus
- mild and self-limiting
***Acute vs Chronic Intravascular Haemolysis
Acute Intravascular Haemolysis:
—> free Haemoglobin released into blood
—> Haemoglobinaemia / Haemoglobinuria (vs haematuria: RBC in urine) / ↓ Haptoglobin level / ↑ Bilirubin level
Chronic Intravascular Haemolysis:
—> Fe deposition in urine / urothelial cells (i.e. renal tubules)
—> Haemosiderinuria (not detected until later phase)
***Rhesus system
- More than 50 antigenic specificities
- Transmembrane ***polypeptide
- Rh antigens high immunogenic
- Main antigenic specificities: D, C, c, E, e
- 2 closely linked genes: D and CcEe (CE, Ce, cE, ce)
- ***RhD: most immunogenic besides ABO system
Rh Ab:
- **Acquired after exposure to Ag that patient lacks (i.e. not pre-existing like in ABO)
- **Warm IgG
- can ***cross placenta
Example:
RhD -ve: no RhD Ag on RBC (of mother)
—> if given RhD +ve RBC transfusion (from 1st Rh+ve baby)
—> high chance to develop Anti-RhD Ab (in mother)
—> problems with subsequent transfusion (esp. for women at reproductive age) (2nd Rh+ve baby)
—> IgG: can cross placenta and cause HDN
***Significance of Rhesus system
- Transfusion reaction:
- due to immune Rh Ab
- usually **extravascular (∵ IgG) destruction of unlike RBCs
- **extravascular haemolysis: Anti-Rh attach to RBC —> when pass through splenic/liver vessels —> **macrophage activated —> RBC destroyed by **reticuloendothelial system
- Hyperbilirubinaemia, Haptoglobin ↓
- ***NOT see Haemosiderinuria / Haemoglobinemia / Haemoglobinuria - HDN due to Anti-D:
- most common HDN in Caucasians (10-13% Rh(D)-ve)
- Rh(D)-ve mother + Rh(D)+ve father —> Rh(D)+ve fetus
- 1st pregnancy not affected but after that mother acquire Anti-RhD Ab
- sensitisation usually occur during labour where fetal RBC enter maternal circulation
—> **Allo-immunisation
- subsequent pregnancies are affected (if fetus is Rh(D)+ve), usually severe
- mother should receive **Rh(D) Ig (i.e. Anti-RhD) as prophylaxis
—> ***destroy any Rh(D)+ve fetal RBC
—> prevent formation of Anti-RhD Ab
All Chinese can practically be considered Rh(D)+ve (only 0.3% Rh(D)-ve)
—> 唔怕阿媽produce Anti-RhD, 因為本身阿媽都係Rh(D)+ve —> 唔會develop Ab
Other blood group systems
Only significant if corresponding Ab is warm reacting
- Duffy, Kidd, Kell, Ss
- Transfusion reaction, HDN
Not significant if it is cold Ab
- MN, Lewis etc.
***Type and Screen (Pre-transfusion compatibility test)
Type: ABO and RhD blood group (睇RBC有咩Antigen)
- by commercially available Ab (i.e. Anti-serum)
- Anti-sera have known blood group specificity Ab e.g. Anti-A, Anti-D etc.
- DAT: RBC + Anti-serum —> Agglutination —> positive result
—> add Anti-human Globulin to augment agglutination reaction
Screen: Antibody screen (睇有咩Antibody)
- Positive (~7%) / Negative
- screen patient’s plasma for Ab against non-ABO but clinically significant blood group Ag
- formation / acquisition of Ab: through Sensitisation / Transfusion
- primary exposure: short duration, moderate response
- secondary exposure: long duration, larger response —> Anamnestic response (Ab titre ↑ greatly —> haemolysis)
- IAT: Recipient Serum + Donor RBC —> Agglutination —> positive result
—> add Anti-human Globulin to augment agglutination reaction
Principles of Blood bank testing
Separate blood into cells + serum
Plasma/Serum:
- Plasma/Serum grouping
- Antibody screen
- Crossmatch
Cells:
- Cell grouping
- Red cell phenotype
- ***Direct antiglobulin test (DAT)
Direct Antiglobulin test (DAT)
Detection of Antibody ***on RBC (see whether RBC are sensitised by Ab) (可以睇RBC有咩Antigen)
- test for presence of Ag-Ab reaction
- If RBC coated by human Ig
—> Anti-human-Ig Ab (AHG reagent) can be added
—> used to agglutinate RBC
Indirect Antiglobulin test (IAT)
Detection of Antibody **in serum (睇有咩Antibody)
- basis of **Ab screening and identification
- Ag-Ab reaction
- Incubate recipient serum (i.e. Ab) with ***donor RBC (i.e. Ag) (usually a panel of RBC is employed)
—> Ab will only coat RBC with corresponding Ag
—> 再add AHG agent to augment reaction
—> agglutination
—> Since Ag of RBC is known —> the specificity of Ab can be deduced
