an agglutinant; (in later use) spec. an agglutinin.

An antibody or other substance that causes agglutination of cells, microorganisms, etc. (see agglutination, n. 7).

an agglutinin that agglutinates cells of other individuals of the same species as that in which it is found.

an agglutinin that causes agglutination of foreign cells, esp. red blood cells of another group or from an animal of another species.

an isoagglutinin which agglutinates red blood cells.