Body Fluids and Circulation · Lesson 2 of 10
ABO and Rh Blood Groups: Compatibility and Pregnancy
“Explain antigens and antibodies, predict red-cell transfusion compatibility and understand Rh sensitisation during pregnancy.”
• Distinguish RBC surface antigens from plasma antibodies. • Determine ABO compatibility for red-cell transfusion. • Explain Rh-positive and Rh-negative phenotypes. • Trace the mechanism of Rh sensitisation and haemolytic disease of the fetus and newborn. • Explain why blood grouping and crossmatching remain necessary in clinical practice.
Why can transfusing the wrong blood be dangerous?
Blood looks broadly similar from one person to another, yet red cells carry surface molecules that the immune system can recognise. A recipient may have antibodies against a donor's red-cell surface antigen. If these antibodies bind compatible antigens, they can promote red-cell clumping and destruction, sometimes causing life-threatening complications. This is why matching blood is about immunology, not the colour or apparent consistency of a sample.
A molecular feature that can be recognised by components of the immune system; A and B blood-group antigens occur on the surface of red blood cells.
An immune protein that binds a particular antigen; in ABO grouping, plasma naturally contains antibodies against the A or B antigen absent from one's own red cells.
The four ABO groups
The ABO system is determined chiefly by two red-cell surface antigens: A and B. Group A cells carry A antigen and group A plasma generally contains anti-B antibodies. Group B carries B antigen and generally has anti-A. Group AB carries both and normally lacks anti-A and anti-B antibodies. Group O lacks both A and B antigens but usually carries both corresponding plasma antibodies. This pattern follows a simple principle: people generally do not maintain naturally occurring antibodies against their own ABO red-cell antigens.
| Recipient group | RBC antigens | Plasma antibodies | ABO red-cell donors |
|---|---|---|---|
| A | A | Anti-B | A, O |
| B | B | Anti-A | B, O |
| AB | A and B | Neither anti-A nor anti-B | AB, A, B, O |
| O | Neither A nor B | Anti-A and anti-B | O only |
In red-cell transfusion, a group O donor's red cells lack A and B antigens and are broadly compatible in the ABO system, while a group AB recipient has neither anti-A nor anti-B antibodies. The familiar phrases 'universal donor' and 'universal recipient' are useful only with this qualification: they refer to ABO compatibility for packed red cells, not to all blood products or all antigen systems. Real transfusions require typing, antibody screening and crossmatching; group O is not automatically safe for every recipient.
The Rh system adds another important antigen
The Rh grouping discussed in this chapter focuses on the D antigen. Rh-positive red cells express the D antigen; Rh-negative red cells do not. The chapter describes most people as Rh positive. Unlike naturally occurring ABO antibodies, clinically important anti-D antibodies usually arise after sensitising exposure to Rh-positive red cells, such as a transfusion or movement of fetal red cells into the maternal circulation. Thus ABO and Rh systems must be evaluated separately.
A person of group A can be A-positive or A-negative; a person of group O can likewise be O-positive or O-negative. A blood label therefore combines two different classification systems. The presence of Rh D antigen is not a sign that a person's blood contains more oxygen, nor does it determine their ABO group.
Rh incompatibility during pregnancy
Consider a pregnant Rh-negative mother carrying an Rh-positive fetus. Fetal and maternal blood normally circulate separately across the placenta rather than directly mixing. Nevertheless, small numbers of fetal red cells can enter maternal circulation, particularly around childbirth and sometimes earlier because of bleeding or procedures. This exposure may stimulate the mother's immune system to make anti-D antibodies; the process is called sensitisation.
In a later Rh-positive pregnancy, maternal anti-D IgG antibodies can cross the placenta and bind fetal red cells. Their destruction may cause fetal anaemia and jaundice after birth, and in severe cases life-threatening illness. The older term erythroblastosis fetalis is used in the chapter; the broader clinical term is haemolytic disease of the fetus and newborn. It is inaccurate to state that every first Rh-incompatible pregnancy is completely safe, since earlier sensitisation can occur.
Preventive anti-D immunoglobulin is administered to eligible Rh-negative pregnant people who have not already formed anti-D, according to medical protocols. The preparation reduces the likelihood that the maternal immune system will become sensitised after fetal red-cell exposure. It can be given during pregnancy and after delivery or sensitising events, not merely after a first delivery. Once immune anti-D has already developed, this prevention does not erase existing sensitisation.
Apply the logic, not just the table
For every proposed red-cell transfusion, ask two questions. First, what antigens would the donated red cells introduce? Second, does the recipient have antibodies that could attack those antigens? For example, group B recipients generally possess anti-A antibodies, so group A red cells are unsuitable. By contrast, group AB recipients do not have those two ABO antibodies, but an Rh-negative AB recipient still needs Rh compatibility considered.
Blood-group tables teach fundamental immunology. Never use a simplified table to select a blood product in real life: laboratories must assess additional red-cell antibodies and product-specific compatibility.
Quick Quiz
Quiz
Which antibodies are expected in the plasma of a group A individual?
A recipient with group O blood ordinarily accepts packed red cells from which ABO group?
Which RBC antigen determines the Rh-positive label in this simplified system?
Why is Rh disease more likely after maternal sensitisation?
Which blood-group statement is unsafe without qualifications?
Why is crossmatching performed even after ABO/Rh typing?
Practice Problems
- Make a four-row ABO antigen–antibody table and state the compatible ABO red-cell donors for each recipient.
- A patient is B-negative. Explain why A-positive red cells are unsuitable.
- Explain why 'universal donor' is a restricted term.
- Describe Rh sensitisation and its significance for a later pregnancy.
- How does preventive anti-D immunoglobulin differ from treating established sensitisation?
A: A antigen, anti-B, donors A/O; B: B antigen, anti-A, donors B/O; AB: A and B antigens, no anti-A/anti-B, donors all ABO groups; O: neither antigen, anti-A and anti-B, donors O.
Key Takeaways
• ABO classification depends on A and B antigens on RBCs and corresponding plasma antibodies. • ABO red-cell donor compatibility must be determined from recipient antibodies, not from the donor label alone. • Rh-positive means the D antigen is present; Rh classification is separate from ABO. • Maternal anti-D IgG after sensitisation can damage fetal Rh-positive red cells. • Clinical transfusions require individual compatibility testing beyond simplified 'universal' rules.