10.5 - Blood Groups of Humans

ABO Blood Group System

Antigens and Antibodies

• Red blood cells carry antigen A, antigen B, both antigens or neither antigen.
• Blood serum contains anti-A antibodies, anti-B antibodies, both antibodies or neither antibody.

Classification of ABO Blood Groups

Blood group	Antigen on red blood cells	Antibody in blood serum
A	Antigen A	Anti-B
B	Antigen B	Anti-A
AB	Antigen A and antigen B	None
O	None	Anti-A and anti-B

Blood Donation Compatibility and Agglutination

• Blood transfusion depends on compatibility between donor and recipient blood groups.
• Incompatible blood causes agglutination (clumping of red blood cells).
• Group O is the universal donor; group AB is the universal recipient.

Recipient	Compatible donor groups
A	A, O
B	B, O
AB	A, B, AB, O
O	O

Rhesus (Rh) Factor

Rh-Positive and Rh-Negative

• Rh-positive: Red blood cells possess the Rhesus factor or antigen D.
• Rh-negative: Red blood cells do not possess the Rhesus factor or antigen D.
• If Rh-positive donor blood mixes with Rh-negative recipient blood, the recipient produces Rhesus antibodies or anti-D antibodies.
• A subsequent dose of Rh-positive blood can cause donor red blood cells to agglutinate and may be fatal.

Pregnancy and the Rhesus Factor

First Pregnancy

• A problem can arise when an Rh-negative mother carries an Rh-positive foetus.
• During the last month of pregnancy, fragments of foetal red blood cells containing antigen D may cross the placenta into the mother’s blood.
• The mother’s white blood cells produce anti-D antibodies, which remain in her circulatory system.
• Their concentration is too low to affect the first child.

Second Rh-Positive Pregnancy

• Existing maternal anti-D antibodies cross the placenta and destroy the foetus’s red blood cells.
• This condition is erythroblastosis fetalis.
• The baby may die before or soon after birth; a less severe case may cause anaemia and mental disability.
• The foetus may require replacement with Rh-negative blood through blood transfusion.

Prevention

• Treating the mother with anti-Rhesus globulins after the first pregnancy stops anti-D antibody formation.