The clumping together by antibodies of microscopic foreign particles, such as red blood cells or bacteria, so that they form a visible pellet-like precipitate. Agglutination is a specific reaction, i.e. a particular antigen will only clump in the presence of its specific antibody; it therefore provides a means of identifying unknown bacteria and determining blood group. When blood of incompatible blood groups (e.g. group A and group B—see abo system) is mixed together agglutination of the red cells occurs (haemagglutination). This is due to the reaction between antibodies in the plasma (agglutinins) and agglutinogens (antigens) on the surface of the red cells.