Blood group systems:

Blood group systems are a classification of blood-based on the presence or absence of antigens on the surface of red blood cells.

Antigen:

An antigen is a molecule that can stimulate an immune response (antibody production etc). These antigens may be proteins or polysaccharides, depending on the blood group system.

Note:

A total of 29 human blood group sisters are now recognized by the International Society of Blood Transfusion (ISBT).