Agglutination during blood transfusion occurs when:

- Red blood cells from the donor have antigens on their surface that are different from the recipient's antigens.

- The recipient's immune system recognizes these foreign antigens and produces antibodies against them.

- The antibodies bind to the antigens on the donor's red blood cells, causing them to clump together (agglutinate).

This reaction can lead to serious complications, including hemolysis (red blood cell destruction) and even death. To prevent this, blood types are carefully matched before transfusion.