Which of the following blood groups will clump when mixed with anti-a antibody serum?

In the ABO blood group system, each blood group is defined by the antigens present on the surface of red blood cells. Blood group A carries the A antigen, blood group B carries the B antigen, blood group AB carries both A and B antigens, and blood group O carries neither.

Anti-A antibody serum contains antibodies that specifically bind to the A antigen. When anti-A serum is added to a blood sample, agglutination (clumping) occurs if the red blood cells carry the A antigen on their surface.

Blood group A has the A antigen, so it will clump with anti-A serum. Blood group AB also has the A antigen (along with B), so it too will clump with anti-A serum. Blood group B has only the B antigen and blood group O has no antigens, so neither of these will clump with anti-A serum.

Therefore, both A and AB blood groups will clump when mixed with anti-A antibody serum.

Exam tip: Remember that anti-A serum reacts with any blood group that carries the A antigen. AB carries both antigens, so it reacts with both anti-A and anti-B sera.