Practice question

Agglutination reaction visual detection of antigen antibody crosslinking forming large lattices precipitating out of suspension forms basis rapid diagnostics usable at bedside without sophisticated equipment. Mechanism relies on multivalent antigen expressed repetitively on particulate carriers like erythrocytes expressing ABO antigens A trisaccharide with terminal N-acetylgalactosamine, Rh D polypeptide 12 transmembrane helices, bacteria displaying O polysaccharide lipopolysaccharide, or inert latex beads coated with purified recombinant protein, crosslinked by multivalent antibodies. IgM pentamer 900 kDa with ten Fab arms particularly efficient due to span 35 nanometers allowing bridging of multiple cells simultaneously causing clumping visible to naked eye. IgG bivalent can agglutinate but requires enhancement media albumin reducing zeta potential electrostatic repulsion negative charge surrounding erythrocytes, or protease bromelain cleaving sialoglycoproteins reducing surface charge. Reaction kinetics obey Heidelberger-Kendall precipitin curve where antibody excess forms soluble small complexes, equivalence zone yields maximal insoluble aggregates settling, antigen excess resolubilizes. Direct agglutination test mixes patient red cells with commercial antisera anti-A, anti-B providing ABO typing within two minutes.