The reaction between antibody and soluble antigen is demonstrated by -
When a soluble antigen companies with its antibody in the presence of electrolyte at a suitable temp & pH the antigen-antibody complex forms an insoluble precipitate. Ref: Ananthanarayan & Parker's textbook of microbiology 9th edition pg:105
Agglutination
Precipitation
Complement fixation test
Hemagglutination test
Immunology