Practice question

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 lipopolysaccharide carbohydrate chain 10 repeats H flagellar flagellin. When antibody encounters erythrocyte expressing antigen Fab arms bind distinct cells crosslinking forming lattice three-dimensional aggregate precipitates due increased size sedimenting visibly clumps. Reaction depends antigen-antibody ratio zone phenomenon Heidelberger-Kendall prozone excess antibody small complexes soluble no visible clumping saturating all epitopes monovalent binding equivalence zone optimal ratio large insoluble lattice precipitates postzone excess antigen again soluble. Low ionic strength saline LISS reduces zeta potential repulsive negative charge erythrocyte enhancing electrostatic attraction sensitivity. Direct agglutination uses native cells ABO grouping anti-A anti-B sera Widal Salmonella Typhi anti-O anti-H agglutinate bacteria indicating typhoid titer >1:160 Rose Bengal brucellosis. Indirect passive agglutination carrier particles latex beads 0.8 micron sensitized sheep