There are four main blood groups, determined by a system called the ABO system. Your own individual blood group is inherited from the genes of both your parents. To understand your blood type, you must first understand that each type is made up of antibodies and antigens. Antibodies are plasma proteins that help your immune system to defend against foreign substances like germs. Antigens are a different type of protein that are found on the surface of a red blood cell. Think of antigens like a jacket, and your body can recognise that jacket as either your own, or as someone else’s and therefore a threat that needs to be fought off. If you are blood group A, you have A antigens on the surface, with anti-B antibodies in the plasma. Those with blood group B have B antigens on the surface with anti-A antibodies. If you are blood group O, you have no antigens, but you have antibodies against both blood groups A and B. Those with blood group AB have A and B antigens, and therefore antibodies against no blood groups. Most people in the UK are blood group O.
There are times that people need a blood transfusion, such as if you’re losing blood in an operation or from an injury or accident. This transfusion comes from a bank of blood that others have donated. It’s been tested and vetted as safe, and it has been categorised into each blood group from the ABO system. You need to have a transfusion that’s compatible with your own blood group, otherwise your immune system will mount a response and attack it, which can be life-threatening. Blood group A can accept blood group A blood, but not group B, as this will be seen as foreign by the anti-B antibodies. The same goes for if you are blood group B, you cannot receive A blood. If you have group AB, you are known as a universal acceptor, as you can receive any blood type as a transfusion. On the other hand, no one has antibodies to blood group O, so this is the universal donor – it can be safely given to everyone. This is important if it’s an emergency and the lab hasn’t yet processed which blood group a patient is. Those with blood group O can only receive blood group O.
In addition to the four blood groups, the red blood cells have another antigen called the Rhesus D antigen. If it is present, then individuals are Rhesus D positive (RhD +ve). If it is not present, they are Rhesus D negative (RhD -ve). Most people in the UK are Rhesus D positive.
Knowing your blood type is important if you are required to get a blood transfusion. In an emergency where you are losing a lot of blood and need this replaced immediately, there may not be time to get your own blood group checked. Therefore the safest blood group to give to others is O negative that's also Rhesus D negative, as there are no antigens on the surface for the recipient to react to. It’s always important that pregnant women know their blood type. This is because the mixing of blood types between mother and child can cause a reaction from the mother – this is if the mother is Rhesus D negative and her growing baby is Rhesus D positive. She will build antibodies that may not affect her first pregnancy, but may mount a response to future pregnancies, and cause a life-threatening condition called Rhesus disease of the newborn. Mothers who are Rhesus D negative will receive immune suppression to prevent this response.
Your GP doesn’t routinely check your blood group. It will be checked in an emergency, before invasive surgery, if you are pregnant and if you are going to give blood. You can have bloods taken privately to test your blood group. Blood groups have sometimes been used in custody battles, if a child has a different blood group to their father. If you are involved in this sort of situation, it’s important to gather all of the facts before drawing conclusions. Genetics is complex, and an expert can take you through the chances of a child having a particular blood group, based on both their mother’s and father’s blood groups. It doesn’t necessarily follow that children have the same blood groups as either parents.