ABO blood types can be determined by agglutination tests that detect A and B antigens on the surface of human red blood cells. When antibodies directed against A antigens (anti-A antiserum) are mixed with blood from a person with Type A blood, agglutination (clumping) will be visible. Likewise, when antibodies directed against B antigens (anti-B antiserum) are mixed with blood from a person with Type B blood, agglutination will be visible. Persons with type AB blood will exhibit agglutination with both anti-A and anti-B antiserum. Because persons with Type O blood lack both A and B antigens on their cells, no agglutination will be visible when their blood is mixed with anti-A or anti-B antiserum. 
 

Agglutination reactions are also used to determine the presence or absence of Rh factor (Rh antigen). Individuals who are Rh positive have Rh antigens on the surface of their red bloods cells. Their blood will agglutinate when mixed with anti-Rh antiserum. This visible reaction is enhanced when the blood is heated slightly on an Rh typing box. Persons who are Rh negative lack Rh factor on their red blood cells. No agglutination will occur when Rh negative cells are mixed with anti-Rh antiserum. 

1. Type A	2. Type B	3. Type AB	4. Type O

5. Rh negative	6. Rh positive	7. Rh typing box