ABO Testing

A small sample of blood is mixed with a drop of a serum containing either anti-A or anti-B antibodies. These antibodies will react with antigen A and antigen B respectively and cause red cells to clump together (agglutination). Thus if this is seen when blood is mixed with anti-A serum the red cells must possess the A antigen. Similarly agglutination with anti-B serum indicates B antigens. If agglutination is seen with both anti-A and anti-B sera the blood group must be AB, whilst no agglutination with either serum indicates group O since the group O red cells possess neither A nor B antigens.