Blood is made up of four major components:
- red blood cells, which carry oxygen around the body cells
- white blood cells, the body’s defence mechanism
- platelets, which help form blood clots
- plasma, where the protein lives.
It is the red blood cells which define which blood group you belong to. On the surface of the red cells there are little markers called antigens; they are so small that they can’t even be seen under a microscope. But, apart from identical twins, each person has different antigens. These antigens are the key to identifying blood types and must be matched in transfusions to avoid serious complications.
The structure for defining blood groups is known as the ABO system. If you have blood group A, then you have A antigens covering your red cells. B means B antigens, while O group has neither and AB has both.
The ABO system also covers antibodies in the plasma which are the body’s natural defence against foreign antigens. So, for example, blood group A has anti-B in their plasma, B has anti-A and so on. To complicate matters a little, group AB has no antibodies and group O has both. If these antibodies find the wrong red blood cells, they will attack them. That’s why giving the wrong ABO blood can be fatal and why group A blood can never be given to a group B patient and vice-versa.
There is also another factor to be considered – the Rh system. Rh antigens can be present in each of the blood groups. Some of us have them and some of us don’t. If the Rh antigens are present then you are Rh positive. A person with A blood group and Rh positive is known as A+, while if the Rh is negative, they are A-. The same applies for B, AB and O.