Agglutinin is an antibody that causes agglutination, the clumping of cells or particles, such as red blood cells, in response to specific antigens. This property is clinically significant in blood typing and transfusion medicine, as it helps determine compatibility between donor and recipient blood types, preventing transfusion reactions. A key example is the presence of anti-A or anti-B agglutinins in individuals with type O blood, which can lead to severe hemolytic reactions if mismatched blood is transfused.