Systematic Veterinary Virology

INTRODUCTION

• The technique involves cutting wells into an agarose solidified in a Petri dish. The wells are filled with antibody or antigen and the dish is incubated. When homologous antigen and antibody diffuse toward each other from the individual wells, a precipitin line will form somewhere between the two wells. Precipitation occurs because the antigen is multivalent i.e., has several antigenic determinants per molecule to which antibodies can bind. Antibodies have at least two antigen binding sites, thus large aggregates or lattices of antigen and antibody are formed. Precipitation/cross-linking will not occur if excess antigen is present or if excess antibody is present. Cross-linking and lattice formation will only occur when antigen and antibody concentrations are optimal. An increasing amount of antigen is added to a constant amount of antibody in solution. This is called the antibody-excess zone (Prozone phenomenon).