The coagglutination phenomenon of Bordet and Gengou involves a reaction between antibody-aggregated fourth component of complement and a receptor on guniea pig red cells.

The coagglutination phenomenon of Bordet and Gengou has been re-investigated. Many of the original observations have been confirmed. However, the explanation of the phenomenon has been found to be more special than that offered by Bordet and Gengou, who envisaged 'entrainment; or enmeshment of guinea pig red cells by lattices formed by antibodies reacting with serum proteins. We found that, so far as human serum is concerned, coagglutination of guinea pig red cells is confined to an activity of the fourth component of complement following its reaction with antibody. The nature of the association between antibody-reacted or antibody-aggregated C4 and the receptor on the guinea pig red cells is still to be elucidated.