Practice question

Precipitation reaction is governed by lattice theory described by Marrack. Small soluble complexes form when one component dominates, remaining invisible. At equivalence zone, ratio of antigen to antibody valencies matches, allowing extensive alternating cross-linking into large three-dimensional network. These macromolecular aggregates become insoluble and precipitate efficiently, yielding maximal turbidity and fastest sedimentation. Antigen excess or antibody excess each saturates binding sites with same species, preventing bridging. Therefore laboratories titrate reactants to approach equivalence for immunodiffusion, quantitative precipitin curves and antisera standardization to achieve strongest visible precipitation.