Cells (such as bacteria, fungus, and erythrocytes) and insoluble particulate antigens can be directly agglutinated by their specific antibodies. The antibody has two Fab arms with which it can bind to antigens on two cells. Likewise many antibody molecules bind with a number of cells to form a lattice.
This lattice formation is seen visually as clumps. Hence, formation of clumps indicates the presence of antigen-antibody binding. Lack of agglutination indicates the absence of antigen-antibody reaction.