NEW Written to fill a gap — ABO/Rh antigens and antibodies, and the three levels of compatibility testing.
Blood groups & crossmatching
Which group carries which antigen, which carries which antibody, and why the donor rule reverses between red cells and plasma.
The ABO system — antigen and antibody
The single most important idea: you carry antibodies against the antigens you do NOT have. Everything else follows from that.
Group
Antigen on the red cell
Antibody in the plasma
A
A
Anti-B
B
B
Anti-A
AB
A and B
NONE
O
NONE
Anti-A and anti-B
Read the table diagonally
AB has every antigen and no antibody — so it can receive any red cells, but its plasma is the most restricted.
O has no antigen and every antibody — so its red cells are safe for anyone, but it can only receive O red cells.
A and B sit in the middle, each carrying the antibody against the other.
These anti-A and anti-B antibodies are naturally occurring — they appear in the first few months of life without any transfusion or pregnancy, probably provoked by gut bacterial antigens. This is why an ABO mismatch causes a reaction on the very first transfusion.
The donor rule reverses for plasma
Product
Universal donor
Universal recipient
Red cells
O negative
AB positive
Plasma / FFP
AB
O
Platelets
Ideally group-identical; A often used as the pooled default
Some plasma is carried over, so ABO still matters
Cryoprecipitate
Small plasma volume — ABO preferred but not essential
—
The logic, so you never have to memorise it
When you give RED CELLS you are giving ANTIGEN. So you must give the group with the fewest antigens — O.
When you give PLASMA you are giving ANTIBODY. So you must give the group with the fewest antibodies — AB.
In an emergency in a woman of childbearing potential, use O negative; in a man or an older woman, O positive is acceptable and conserves the scarce O negative stock.
The Rhesus system
D is the most immunogenic antigen of the Rh system (C, c, D, E, e). "Rh positive" means D antigen present.
About 85% of people are RhD positive.
Anti-D is NOT naturally occurring. It is an immune antibody, produced only after exposure — transfusion, or fetomaternal haemorrhage.
Anti-D is IgG, so it crosses the placenta — hence haemolytic disease of the newborn. Anti-A and anti-B are IgM and do not.
Prophylactic anti-D immunoglobulin is given to RhD-negative women after any sensitising event (delivery, miscarriage, amniocentesis, abdominal trauma, antepartum haemorrhage) and routinely in the third trimester.
Anti-A / anti-B
Anti-D
Class
IgM
IgG
Origin
Naturally occurring
Immune — needs prior exposure
Crosses placenta
No
Yes
Complement fixing
Yes — powerfully
Poorly
Site of haemolysis
Intravascular — severe, immediate
Extravascular — splenic, delayed
Reaction on first exposure
Yes
No — sensitises only
This table explains why an ABO error kills within minutes while an Rh error typically causes a delayed haemolytic reaction 5–10 days later, with a falling haemoglobin and a rising bilirubin.
The three levels of compatibility testing
Test
What it does
Time
1 · Group and save (type and screen)
Determines ABO and RhD, then screens the patient's plasma against known cells for atypical antibodies
~15–30 min
2 · Electronic issue
If the group is known from two samples and the antibody screen is negative, the computer selects compatible units with no laboratory crossmatch at all
~5–10 min
3 · Full serological crossmatch
Patient plasma is physically mixed with donor cells and observed for agglutination. Needed when the antibody screen is positive
~45–60 min, longer if antibodies must be identified
In an emergency
O negative, uncrossmatched — immediate, held in the fridge in theatre or ED.
Group-specific, uncrossmatched — about 10–15 minutes. Safer than O negative once the group is known, and it conserves stock.
Fully crossmatched — 45–60 minutes.
A group and save is NOT a crossmatch. It means the lab holds a sample and knows the group — blood can then be issued quickly, but none is reserved.
Direct and indirect Coombs
Direct antiglobulin test (DAT / direct Coombs): detects antibody already bound to the patient's red cells. Used to investigate a suspected transfusion reaction, autoimmune haemolytic anaemia and haemolytic disease of the newborn.
Indirect antiglobulin test (indirect Coombs): detects free antibody in the patient's plasma. This is the basis of the antibody screen and the crossmatch, and of antenatal screening.
Memory hook: Direct = on the cell. Indirect = in the serum.
Acute haemolytic transfusion reaction
Almost always ABO incompatibility from a clerical or identification error — the wrong patient, the wrong sample, or the wrong unit. It is a checking failure, not a laboratory one.
Awake: fever, rigors, loin and chest pain, restlessness, a sense of impending doom, dark urine.
Under anaesthesia the only signs may be unexplained hypotension, haemoglobinuria, and generalised oozing from the surgical field as DIC develops — the patient cannot tell you they feel dreadful.
Management: STOP the transfusion immediately. Keep the line with new giving set and saline. ABC, aggressive fluid to maintain urine output, treat hyperkalaemia and DIC, and return the unit and a fresh sample to the laboratory. Check patient identity against the unit. Report as a serious incident.