What is the clumping together of RBCs by antibodies?

The clumping together of red blood cells by antibodies is called agglutination. It occurs when antibodies bind to specific antigens on the surface of red blood cells, causing them to stick together. This can be seen as a visible reaction in a blood test called a hemagglutination assay, which is used to determine blood type or to detect the presence of certain antibodies in the blood.