In biology, "agglutination" can mean a couple of things:

  1. The clumping together of cells due to the binding of agglutinin (a protein) molecules on each cell's surface.

  2. The clumping together of two microbial organisms of the same species for the purpose of sexual reproduction. This is often conducted by means of a carbohydrate on one organism and a protein on the other, resulting in a glycoprotein (agglutinin) that helps helps them stick together.

From the science dictionary at