The reaction between antibody and soluble antigen is demonstrated by

When a soluble antigen combines with its antibody in the presence of an electrolyte (NaCl) at a suitable temperature and PH, the antigen-antibody complex forms an insoluble precipitate at a suitable temperature and pH, the antigen-antibody complex forms an insoluble precipitate.When, instead of sedimenting, the precipitate remains as floccules, the reaction is known as flocculation. Ananthanarayana 7e pg:94