Blood Groups

Every donor's blood belongs to one of the four main groups A, B, AB and O, known as the ABO system. These groups depend for their identification on the presence or absence of chemical substances called antigens on the person's red blood cells.

After the ABO system, the most important is the Rhesus system or RH system. The RH groups are more complicated, but most donors can be simply classified as Rh Positive or Rh Negative.

As each system is inherited independently of the other, it means that there are eight main blood groups: A+, A-, B+, B-, AB+, AB-, O+ and O-.

The Donor/Receiver chart below shows the compatibility of different blood groups. From this you can see that O- blood is the universal donor while AB+ is the universal receiver.

Blood Group	Donate to	Receive from
A+	A+, AB+	A+, A-, O+, O-
O+	O+, A+, B+, AB+	O+, O-
B+	B+, AB+	B+, B-,O+, O-
AB+	AB+	All groups
A-	A+, A-, AB+, AB-	A-, O-
O-	All groups	O-
B-	B+, B-, AB+, AB-	B-, O-
AB-	AB+, AB-	AB-, A-, B-, O-

How Are Blood Groups Inherited?

Blood groups for each individual are determined by genes which are inherited from both parents. Genes for the Rh negative and O groups from one parent are masked by the presence of Rh positive and A or B genes from the other parent. Thus people who are apparently A or B Rh positive may also carry genes for the O and Rh negative blood groups which can be inherited by their children.

Are There Any Other Blood Groups? There are 15 other blood group systems - in all almost 400 antigens have been discovered. However, most of these do not normally require matching for transfusion purposes.

Rh blood grouping

The RhD antigen is also important in determining a person's blood type. The terms "positive" or "negative" refer to either the presence or absence of the RhD antigen irrespective of the presence or absence of the other antigens of the Rhesus system. Anti-RhD is not usually a naturally occurring antibody as the Anti-A and Anti-B antibodies are. Cross-matching for the RhD antigen is extremely important, because the RhD antigen is immunogenic, meaning that a person who is RhD negative is very likely to make Anti-RhD when exposed to the RhD antigen (perhaps through either transfusion or pregnancy). Once an individual is sensitised to RhD antigens their blood will contain RhD IgG antibodies which can bind to RhD positive RBCs and may cross the placenta.