Antibodi dalam serum, yang bila tercampur dengan antigen homolognya, menyebabkan elemen antigen saling melekat membentuk gumpalan. Juga substansi lain, misalnya lektin, yang mampu menimbulkan aglutinasi.
An agglutinin is a substance that causes particles to coagulate to form a thickened mass . Agglutinins can be antibodies that cause antigens to aggregate by binding to the antigen-binding sites of antibodies. Agglutinins can also be any substance other than antibodies such as sugar-binding protein lectins. Agglutinins work by clumping on particles causing the particles to change from fluid-like state to thickened-mass state. When an agglutinin is added to a uniform suspension of particles such as bacteria or blood, agglutinin binds to the agglutinin-specific structure on the particle causing the particles to aggregate and fall to the bottom leaving a clear suspension. This phenomenon known as agglutination is of great importance to the medical world as it serves as a diagnostic tool.