Blood Groups

ABO System

• Blood cells have proteins called antigens on their surface.
• The ABO system classifies blood based on the presence of antigen A and/or antigen B.
• Depending on the antigen present, red blood cells are assigned to one of four blood groups: A, B, AB, and O.
• When blood transfusions are given, the incoming blood group must match the blood group of the recipient. If not, the blood may clump in the recipient.

Blood group:	Antigen on red blood cell:	Can receive blood from:	Can donate blood to:
A	A	A, O	A, AB
B	B	B, O	B, AB
AB	A and B	A, B, AB, O	AB
O	No antigen	O	A, B, AB, O

Rhesus Factor

In addition to A and B antigens, some red blood cells have the rhesus factor (antigen D).

• Cells with antigen D are rhesus positive (Rh⁺).
• Cells without antigen D are rhesus negative (Rh⁻).
• Example: blood group A⁺ = antigen A & the rhesus factor.

Rhesus status:	Can donate to:	Can receive from:
Rh⁺	Rh⁺	Rh⁺, Rh⁻
Rh⁻	Rh⁺, Rh⁻	Rh⁻

Rhesus Factor in Pregnancy

• A rhesus negative mother with a rhesus positive baby may experience complications.
• During the first pregnancy, no issues usually arise.
• However, any further rhesus positive babies may cause the mother to release antibodies.
• These diffuse through the placenta to the foetus where they can damage the red blood cells, leading to anaemia, brain damage, or death in the foetus.
• Screening of pregnant women is commonly carried out and treatments can be given to prevent the mother producing antibodies against the baby's blood.