Practice question

Precipitation requires formation of large lattice through cross-linking where bivalent or multivalent antibodies bridge polyvalent antigens. Intact IgG with two Fab arms and F(ab')2 dimer retain bivalency, enabling network extension and insoluble aggregate generation. Whole antibody adds Fc but remains precipitable. In contrast, Fab fragment obtained by papain digestion is monovalent, possessing single binding site. It can bind epitope and block it, but cannot connect two antigen molecules to build lattice. Resulting complexes stay small and soluble, invisible in gel. Therefore Fab is used to inhibit agglutination or precipitation, not promote it.