Agglutination occurs in blood transfusion when the

Agglutination is the clumping together of red blood cells, which can block blood vessels and be fatal. It occurs during a blood transfusion when the antigens on the donor's red blood cells react with the antibodies in the recipient's plasma. In other words, agglutination happens when the antigen-antibody combination between donor and recipient is incompatible.

In the ABO blood group system, each blood group carries specific antigens (agglutinogens) on the surface of its red blood cells and specific antibodies (agglutinins) in its plasma:

When a donor's antigen meets a matching antibody in the recipient (for example, antigen A meets anti-A antibody), the antibodies bind to the red blood cells and cause them to clump. This is agglutination.

Race has no bearing on blood compatibility. Same antibodies do not fuse with each other to cause a problem. Being in the same blood group actually ensures compatibility, not agglutination.

The key principle: always match donor antigens against recipient antibodies. Incompatibility between them is what triggers agglutination.