ABO blood groups

Red blood cells are not smooth: they carry chemical markers on their surface that medicine calls antigens. The ABO system looks at only two of those markers, A and B, along with the antibodies that recognize them, and it sorts people into four main groups: A, B, O and AB.

So what puts the markers there? Enzymes with a long name, the glycosyltransferases, lay down chains of sugar on the surface of the cell. Think of a sugar tag that an enzyme sticks onto the red blood cell.

The instructions for making those enzymes are written in a gene, the ABO gene, which is found on chromosome 9. That gene comes in several versions, called alleles, and they do not all work the same way: the O versions are inactive, because they produce no working enzyme.

There is something else, and it shows up on its own. The blood also carries antibodies, the proteins that recognize those markers. In an immunocompetent person, meaning someone whose defense system is already formed, the anti-A and anti-B antibodies start to appear at around six months of age.