ABO Blood Groups Flashcards
Name and construct antigens on the RBC in the ABO system: A, B, and H antigens
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Given phenotypic expression of ABO antigens, predict ABO blood types of offspring from various mating combos.
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Forward typing
Use commercial anti-sera (Ab) against patients RBCs to detect blood group antigen
Reverse typing
Use patient sera against commercial RBCs (A1 and B cells) to see which Ab the patient produces
Word of the day
Bombay
- Rare phenotype first described in Bombay, India where pt does not express A, B, or H antigen
- These pt still have ABO genes that can be transferred to offspring but just can’t express them on RBC bc no H antigen to lock onto
Parabombay
- These pt have no H Ag on RBCs but may express small amounts of A or B Ag on RBC surface bc FUT2 generates small amounts of secrete A or B Ag that can absorb onto RBC (type I chain)
- Or caused by FUT1 that makes fucosyltransferase with very low activity
Type I substances
Secretions, can be absorbed onto RBCs, FUT2
Type II substances
Directly linked to RBC, FUT1
On which chromosome is the ABO gene found?
Chromosome 9
What does the ABO gene encode?
Encodes a glycosyltransferase that adds an N-terminal sugar to the end of H Ag attached to RBC
A transferase adds which sugar to H Ag?
N-acetyl-galactosamine (GalNAc)
B transferase adds which sugar to H Ag?
Galactose
If a stop codon is within the ABO gene, what does this imply?
- No transferase is created, thus no sugar added to Type II substance
- Patient is Type O
The H gene is found on which chromosome?
Chromosome 19
What does the H gene encode?
Fucosyltransferase that adds a fucose to outer-most sugar of a Type I or Type 2 precursor chain
T/F
The more H substance transferred to an A or B antigen, less detectible the H Ag
True
List the H Ag detectability in each blood type from greatest to lowest detectability
O > A2 > B > A2B > A1 > A1B
Why is H Ag not easily detectable in A1 or A1B patients?
- Because almost all H Ag sites are converted to A1 or B, thus serologically masking H Ag
- A1 and H have an inverse reciprocal relationship (more A1, less H)
T/F
Type 2 chain precursor can express A or B without conversion to H Ag
False
Must convert to H Ag before A or B can be expressed
T/F
As patients mature, Type 2 substance can become more branchy
True
Subgroup
When a pt may appear a weaker version of the blood type due to fewer branchy H Ag transferred to A or B
What would cause a patient to form an antibody against anti-branchy H?
If only linear H Ag is transferred to A or B, then the branchy H may appear foreign if introduced through transfusion
List the 2 most common A phenotypes
- A1 = 80-90%
- A2
Describe the A1 phenotype
- Branched A Ag (means that A attached to branched H Ag)
- More A1/RBC than A2/RBC
- Positive with anti-A Ab
- Positive with anti-A1 lectin
- Highly functioning A transferase transfers N-terminal sugar to branchy or linear H Ag no problem
- Most common Ag expression of A
Describe the A2 phenotype
- Linear A antigens (means that A attached to linear H Ag)
- Fewer A2/RBC than A1/RBC
- Positive with anti-A Ab
- Negative with anti-A1 lectin
- Weaker A transferase only transfers N-terminal sugar to linear H Ag
- Second most common expression of A
anti-A1 lectin
- Plant-derived compound with the purpose of distinguishing between A1 and A2 blood types
- Positively reacts with A1 and negatively reacts with A2 bc it detects branchy substances
T/F
B subgroups are more common than A subgroups
False
A subgroups (20% of time) are more common than B subgroups
Which A subgroups weakly agglutinate with anti-A and/or anti-A,B?
- A3
- Aend
- Ax
Which A subgroups show no agglutination and must be detected by elution testing?
- Am
- Ay
- Ael
A3 subgroup
- Mixed-field appearance (some parts agglutinate and some do not, appearing smoky)
- 35k Ag
- May make anti-A1 and secrete A Ag
Aend subgroup
- 10% RBCs hav enough A Ag to agglutinate serologically
- About 3,500 Ag
Ax subgroup
- Does not agglutinate with anti-A
- Does agglutinate with anti-A,B
- 4k Ag
- Almost always have anti-A1 in serum
Am
- Do not react strongly serologically
- No anti-A1 in serum
- Have detectable amounts of A enzyme in serum
- 200-300 Ag
Ay
- Does not agglutinate serologically with anti-A
- Does secrete A Ag in saliva
- Recessive
- No anti-A1
Ael
- Ag presence only detectible by adsorption/elution procedures
- Have Ab that reacts with A1 and sometimes A2 cells
- Only H substance in secretors and no A Ag in saliva
B subgroup
- Very efficient transferase activity
- Strong serological activity
- Detected by bandeirea simplicifolia lectin
- 600k-800k Ag on RBC
B3 subgroup
- Mixed field agglutination with anti-B and anti-A,B
- Weak transferase
- Does not make anti-B
Bx subgroup
- Reacts weakly with anti-B and anti-A,B
- NO detectible amounts of transferase
- Makes weakly reactive anti-B Ab
Bm subgroup
- Detectible transferase but does not work well
- Normal amount of B Ag on secretions
- No anti-B detected (more freq in Japan)
Bel subgroup
- Must be detected by adsorption and elution studies
- Weak anti-B may be present
- B Ag not in secretions
Why is there no B2 subgroup?
Bc no anti-B2 lectin exists to detect B attached to branchy H Ag
Which lectin detects A1?
Dolichos biflorus
Which lectin detects B?
Bandeiraea simplicifolia
Which lectin detects H?
Ulex europaeus
Describe Cis-AB anomaly
- Rare splicing anomaly
- A and B transferases are inherited on the same chromosome
- Can pass AB blood type with O mate and can possibly produce type O children
Describe B(A) pt anomaly
- Mainly B antigen with some A present
- Transferase prefers to transfer galactose but can also transfer GalNAc
- Genetic variant of B gene
- Polyclonal anti-sera
Describe acquired B pt anomaly
- If patient is originally A1 but has colon/rectal cancer, intestinal obstruction, gram-neg septicemia, then this can cause breakage of Ag to look like B Ag in a few RBCs
- Mostly apparant with acified anti-sera
Give example of acquired B anomaly
If pt is infected with E.coli, the bacteria have a deacetylating enzyme that removes the acetyl group from the A sugar, leaving a sugar that resembles D-galactose (appearance B)
What does acquired B anomaly look like serologically?
- anti-A: 4+
- anti-B: 1+
- anti-A, B: 4+
Why is the ABO system important?
- ABO system is the ONLY system where reciprocal (antithetical) Ab are consistent and predictable
- Ab present in human plasma that have not been exposed to other human Ag (naturally occurring)
