Practice question

Ouchterlony double diffusion places antigens and antisera in wells cut in agarose, allowing radial diffusion. Precipitation arcs form where equivalence is reached. Pattern comparison reveals antigenic relationships. Identity yields fused continuous arc because antibodies recognize identical determinants and precipitation fronts merge. Partial identity produces fused arc with spur. Non-identity occurs when antigens are structurally unrelated, recognized by different antibody populations; their precipitation lines cross completely without fusion, forming intersecting lines shaped as X. Each line corresponds to distinct immune system, demonstrating absence of cross-reactivity and confirming antigens share no common epitopes.