If a blood sample agglutinates with both anti-A and anti-B, what type of blood is it?

A blood sample that agglutinates with both anti-A and anti-B indicates the presence of both A and B antigens on the surface of the red blood cells. In the ABO blood group system, type AB blood has both antigens present. Therefore, when anti-A serum is introduced, the A antigens will react and cause agglutination. Similarly, the B antigens will react and cause agglutination with anti-B serum.

Type O blood would not agglutinate with either anti-A or anti-B because it has neither A nor B antigens. Type A blood only has A antigens and would agglutinate with anti-A serum, but not with anti-B. Type B blood only has B antigens and would agglutinate with anti-B serum, but not with anti-A. Thus, the observation of agglutination with both anti-A and anti-B definitively identifies the sample as type AB blood.