10.5 Blood Groups of Humans
Learn how human blood is classified into ABO groups, how blood transfusion compatibility works, and how the Rhesus factor can cause complications during pregnancy.
Learning Objectives
- State the antigens and antibodies present in blood groups A, B, AB, and O.
- Determine the compatibility of donor and recipient blood groups during blood transfusion.
- Explain the Rhesus factor (Rh-positive and Rh-negative) and the reaction caused by incompatible Rh blood mixing.
- Describe the mechanism and consequences of erythroblastosis fetalis in an Rh-negative mother carrying an Rh-positive foetus, including its prevention.
The ABO Blood Groups and Rhesus Incompatibility in Pregnancy
A visual summary of the antigens and antibodies in each ABO blood group, transfusion compatibility, and the stepwise mechanism of erythroblastosis fetalis.
Blood group A
Antigen A on red blood cells; anti-B antibody in serum.
Blood group B
Antigen B on red blood cells; anti-A antibody in serum.
Blood group AB
Antigen A and antigen B on red blood cells; no antibodies in serum.
Blood group O
No antigens on red blood cells; anti-A and anti-B antibodies in serum.
Short Explanation
What determines a blood group?
Human blood is classified into four groups — A, B, AB and O — based on the antigens on the surface of red blood cells and the antibodies in the blood serum.
Blood transfusion compatibility
Blood donation and transfusion depend on the compatibility of blood groups between donor and recipient. Incompatible transfusions cause the recipient's red blood cells to experience agglutination.
Universal donor and recipient
Blood group O is the universal donor because it has no antigens on its red blood cells. Blood group AB is the universal recipient because it has no antibodies in its serum.
Rhesus factor
The Rhesus factor is another antigen found on the surface of red blood cells, also known as antigen D. Individuals with antigen D are Rh-positive; those without it are Rh-negative.
Erythroblastosis fetalis
When an Rh-negative mother carries an Rh-positive foetus, foetal antigen D triggers the mother to produce anti-D antibodies. In a subsequent Rh-positive pregnancy, these antibodies destroy the foetal red blood cells, causing erythroblastosis fetalis.
Try to Answer First
Answer in your mind, then press “Check Answer”.
Which ABO blood group is known as the universal donor, and why can it be donated to all ABO groups?
What condition occurs when an Rh-negative mother's anti-D antibodies destroy the red blood cells of a second Rh-positive foetus?
How can erythroblastosis fetalis be prevented in subsequent pregnancies?
Common Mistakes
- Confusing the location of antigens and antibodies: thinking antibodies are on the red blood cells and antigens are in the blood serum, when in fact antigens are on red blood cells and antibodies are in the serum.
- Assuming blood group AB has both anti-A and anti-B antibodies; blood group AB has no antibodies in its serum.
- Assuming blood group O has both antigens; blood group O has no antigens on its red blood cells.
- Believing erythroblastosis fetalis occurs when the mother is Rh-positive and the baby is Rh-negative; it only occurs when the mother is Rh-negative and the foetus is Rh-positive.
- Thinking the first Rh-positive baby dies of erythroblastosis fetalis; typically the first baby survives because the concentration of anti-D antibodies produced is not high enough.
Concept Misunderstandings
Blood group O can receive blood from any blood group because it is a universal donor.
Blood group O can only receive blood from blood group O because its serum contains both anti-A and anti-B antibodies, which will agglutinate any blood cells containing antigen A or antigen B.
The mother's blood and foetal blood mix completely during the entire pregnancy.
Normally, maternal and foetal blood do not mix directly; only fragments of foetal blood cells cross the placenta, usually during the last month of pregnancy or during delivery.
Anti-Rhesus globulin is given to cure the sick second baby.
Anti-Rhesus globulin is given to the mother after the first pregnancy as a preventative measure to stop her from producing anti-D antibodies.
Summary
- ABO blood groups are determined by Antigen A and Antigen B on red blood cells and anti-A and anti-B antibodies in blood serum.
- Incompatible transfusions cause agglutination (clumping of red blood cells). Group O is the universal donor, and group AB is the universal recipient.
- An individual with antigen D is Rh-positive, while one without it is Rh-negative.
- When an Rh-negative mother conceives an Rh-positive foetus, foetal antigen D induces maternal production of anti-D antibodies.
- In a subsequent Rh-positive pregnancy, maternal anti-D antibodies cross the placenta, destroying foetal red blood cells and causing erythroblastosis fetalis. This is prevented by administering anti-Rhesus globulins after the first pregnancy.
Short Activity
Objective: test your understanding of ABO blood groups, transfusion compatibility and the Rhesus factor.
A. Objective Quiz
Which of the following blood groups has no antibodies in its blood serum?
What medical intervention is administered to an Rh-negative mother after delivering her first Rh-positive child to prevent erythroblastosis fetalis in future pregnancies?
B. Fill in the Blanks
If incompatible blood is transfused into a recipient, the recipient's red blood cells will experience .
An individual whose red blood cells possess the Rh factor or antigen D is classified as .
C. Matching / Drag and Drop
Drag each blood group or Rhesus condition to its correct description. On a phone, tap a choice first, then tap the matching box. Tap the Answer Choices area to return a selected item.
C. Match the Blood Group or Rhesus Condition to Its Description
Write the correct term next to each description.
| Description | Term |
|---|---|
| 1. Has no antigens on red blood cells and can donate to all ABO groups. | |
| 2. Contains both antigen A and antigen B, and can receive blood from all ABO groups. | |
| 3. A condition where maternal anti-D antibodies destroy the red blood cells of an Rh-positive foetus. | |
| 4. Red blood cells that do not possess antigen D. |
Answer choices: Blood group O • Blood group AB • Erythroblastosis fetalis • Rh-negative
