Agglutination reactions are based on:
Agglutination depends multivalent antibody capable crosslinking particulate antigens particles larger soluble proteins into large aggregates visible macroscopically naked eye low magnification facilitating rapid low-cost diagnostics bedside. IgM pentameric 900 kDa ten Fab arms arranged star shape span 30 nm ideal bridging IgG bivalent two Fab flexible hinge enabling simultaneous binding distinct particles. Antigen presents repetitive surface array ABO blood group antigens A trisaccharide GalNAc alpha1-3(Fuc alpha1-2)Gal B Gal alpha1-3(Fuc alpha1-2)Gal erythrocyte glycophorin bacteria O lipopol
Ref: Janeway Immunobiology Agglutination Mechanism Chapter 14 Lattice; NCBI Clinical Microbiology Agglutination Tests Principles; Alberts Antigen-Antibody Lattice Formation Theory Kinetics.