Physiology chapters
Blood groups and transfusion
The ABO and Rh blood group systems, which antigens and antibodies each group has, universal donors and recipients, haemolytic disease of the newborn, cross-matching, and what goes wrong in a transfusion reaction.
- ABO groups depend on A and B antigens on red cells; the plasma has natural antibodies (IgM) against the antigens the person lacks.
- Group O has no A or B antigens (universal donor of red cells); group AB has no anti-A or anti-B (universal recipient).
- Rh: antibodies to D appear only after exposure; an Rh-negative mother with an Rh-positive baby can be sensitised, causing haemolytic disease of the newborn in later pregnancies, which anti-D prevents.
On this page
Overview
Red cells carry antigens on their surface. If blood with foreign antigens is given to someone whose plasma contains matching antibodies, the red cells clump (agglutinate) and are destroyed. This is the basis of blood groups.
The two most important systems are ABO and Rh.
The ABO system
The antigens (agglutinogens) are A and B. They are inherited as alleles A, B and O; A and B are co-dominant, O is recessive.
| Group | Antigen on red cells | Antibody in plasma | Can receive red cells from |
|---|---|---|---|
| A | A | Anti-B | A, O |
| B | B | Anti-A | B, O |
| AB | A and B | None | A, B, AB, O |
| O | None | Anti-A and anti-B | O only |
Group O red cells have no A or B antigen, so they are the universal donor of red cells. Group AB plasma has no anti-A or anti-B, so AB people are universal recipients of red cells.
For plasma transfusion the rule is reversed: AB plasma is the universal donor plasma (no antibodies), and group O plasma can only go to group O.
The antibodies
- They are natural: they appear without exposure to foreign blood, within the first few months of life, probably triggered by similar antigens in food and gut bacteria.
- They are mainly IgM, so they do not cross the placenta well. This is why ABO mismatch between mother and baby is usually mild.
In Pakistan, as in much of South Asia, group B is common; the exact frequencies vary by region.
Inheritance
A child inherits one allele from each parent. A group O child must have received an O allele from each parent. Two group O parents can have only group O children.
The rare Bombay phenotype (hh) lacks the H substance on which A and B are built; these people test as group O but have anti-H and can receive only Bombay blood.
The Rh system
The main Rh antigen is D.
- Rh-positive: D antigen present. Most people in Pakistan are Rh-positive.
- Rh-negative: no D antigen.
Unlike ABO, anti-D antibodies are not natural. An Rh-negative person makes them only after exposure to Rh-positive red cells, through transfusion or pregnancy.
Anti-D antibodies are IgG, so they cross the placenta.
The key differences from ABO: Rh antibodies are acquired, not natural, and they are IgG, so they can harm a fetus.
Haemolytic disease of the newborn (erythroblastosis fetalis)
How it happens:
- An Rh-negative mother carries an Rh-positive baby (D antigen from the father).
- At delivery (or miscarriage, abortion, bleeding, amniocentesis), fetal red cells enter her blood.
- She makes anti-D (IgG) antibodies.
- In a later pregnancy with an Rh-positive baby, these antibodies cross the placenta and destroy the baby's red cells.
What you see in the baby: anaemia, jaundice, an enlarged liver and spleen, many immature red cells (erythroblasts) in the blood, and in severe cases heart failure with generalised swelling (hydrops fetalis). High bilirubin can damage the brain (kernicterus).
The first Rh-positive baby is usually not affected, because the mother is sensitised mostly at delivery and her first response is slow and mainly IgM.
Prevention
Anti-D immunoglobulin is given to the Rh-negative mother (who is not already sensitised):
- Within 72 hours of delivery of an Rh-positive baby.
- After any event that may let fetal blood into her circulation.
- Routinely in later pregnancy in many countries.
It destroys fetal Rh-positive cells before her immune system can respond. Follow local obstetric guidance for exact doses and timing.
Treatment of the baby
Phototherapy and, in severe cases, exchange transfusion with Rh-negative blood.
Blood grouping and cross-matching
- Grouping (typing): the patient's red cells are mixed with anti-A, anti-B and anti-D sera; clumping shows which antigens are present. Reverse grouping tests the patient's plasma against known A and B cells.
- Antibody screen: checks the patient's plasma for other unexpected antibodies.
- Cross-match: the patient's plasma is mixed with the donor's red cells to make sure they do not react. This is the final check before transfusion.
Transfusion reactions
Acute haemolytic reaction
How it happens: ABO-incompatible blood is given, usually because of a clerical error (wrong patient or wrong sample). The recipient's IgM antibodies bind the donor cells, activate complement, and destroy them inside the vessels.
What you see: within minutes: fever, chills, back pain, low blood pressure, shock, dark urine (haemoglobinuria), widespread clotting (DIC) and acute kidney injury.
Action: stop the transfusion at once, keep the line open with saline, support circulation, and report and check identities. Follow local transfusion protocols.
Other reactions
- Febrile non-haemolytic: fever from donor white cell products; common and usually mild.
- Allergic: itching and hives; rarely anaphylaxis (for example in IgA-deficient patients).
- Delayed haemolytic: days later, from antibodies to minor antigens.
- Transfusion-associated circulatory overload, and infections.
Expect: the antigens and antibodies of each ABO group, universal donor and recipient, the IgM nature of ABO antibodies, the acquired IgG nature of anti-D, the mechanism and prevention of haemolytic disease of the newborn, what a cross-match tests, and the features of an acute haemolytic transfusion reaction.
Quick revision
- A: A antigen, anti-B. B: B antigen, anti-A. AB: both antigens, no antibodies. O: no antigens, both antibodies.
- O: universal red cell donor. AB: universal red cell recipient.
- ABO antibodies: natural, IgM. Anti-D: acquired, IgG.
- HDN: Rh-negative mother, Rh-positive baby, usually a second pregnancy.
- Anti-D to the mother within 72 hours of a sensitising event.
- Acute haemolytic reaction: ABO mismatch, usually a clerical error.
Written by The House Job from standard physiology references (Guyton and Hall Textbook of Medical Physiology; BRS Physiology).
FAQ
Why is group O called the universal donor?
Group O red cells have no A or B antigens, so they are not attacked by anti-A or anti-B antibodies in the recipient.
What causes haemolytic disease of the newborn?
Anti-D IgG antibodies from a sensitised Rh-negative mother cross the placenta and destroy the red cells of an Rh-positive fetus.
Why is the first Rh-positive baby usually unaffected?
The mother is usually sensitised at the first delivery, so antibodies are not yet present during that pregnancy.
What does a cross-match test?
Whether the patient's plasma reacts with the donor's red cells.