Why have type B red corpuscles clumped together when mixed with type A plasma?

Type B red blood cells clump together when mixed with type A plasma because of the presence of anti-A antibodies in the type A plasma. These antibodies recognize and bind to the A antigens on the surface of type A red blood cells, leading to agglutination. Since type B red blood cells do not have A antigens, the anti-A antibodies target them, causing the clumping reaction. This is a critical aspect of blood type compatibility in transfusions.