Derived from the German word "to glue." The process by which cells are made to adhere to one another, usually through the actions of an agglutinin, such as an antibody or a lectin. Certain viruses and bacteria also are capable of agglutinating blood cells. Many agglutinins, particularly the food lectins, are blood type specific. Certain foods clump only the cells of one blood type, but do not react with the cells of another type.