The reaction between antibody and solouble antigen is demonstrated by

When a soluble antigen reacts with its antibody in the presence of electrolytes (NaCl) at an optimal temperature and pH, the antigen-antibody complex forms an insoluble precipitate and it is called precipitation.  The precipitate usually sediments at the bottom of the tube.  Precipitation may occur in liquid media or in gels such as agar, agarose or polyacrylamide Ref : textbook of microbiology, C P Baveja 5th edition pg no:106