Easily the most common blood group system in the world is tha ABO naming system. This system refers to the different antigens on the surface of the red blood cells, also called erythrocytes. An antigen is a molecule (usually a protein) that causes an immune response when it enters the body. Cells present antigens on their surface – this is why invading bacteria cells cause an immune response.
There are two different antigens that can be presented on the cell surface of an erythrocyte and these antigens are labelled A and B. However there are four different blood types – this is because individuals red blood cells can either have A, B, neither or both.
Therefore the four blood groups are A B O AB
But the antigens comprise only half of the story – their second half is the antibodies, which attach to the antigens to form an antigen-antibody complex. Antibodies are made by white blood cells (Leukocytes). There are three types of Leukocytes – Phagocytes, B Lympocytes and T Lympocytes.
B Lympocytes are the type which make antibodies and are very specific – each cell will only create one type of antibody against one non-self antigen. An individual will however not have antibodies against self-antigens, i.e. the antigens on ones own cells. So if you have blood group A you will have antibodies against antigen B but not against A and vice versa. If you are blood group O you will have antibodies against both A and B antigens and if you’re AB you will have antibodies against neither.
This is very important for blood transfusion as If you give someone a transfusion containing the wrong blood type then their immune system may attack the red blood cells. This causes agglutination (sticking together of the blood cells).
Whether agglutination will occur depends on the patients blood group and the blood type given to them. For example someone with blood type AB can recieve blood of all types as they have antibodies against neither antigen. On the contrary someone of blood group O can only recieve blood of type O as they have antibodies against both antigens.