Precipitin reaction
The precipitin reaction provided the first quantitative assay for antibody. The precipitin reaction is based upon the interaction of antigen with antibody leading to the production of antigen-antibody complexes.
To produce a precipitin reaction, varying amounts of soluble antigen are added to a fixed amount of serum containing antibody. As the amount of antigen added:
The small, soluble immune complexes formed in vivo in the zone of antigen excess can cause a variety of pathological syndromes.
Antibody can only precipitate antigenic substrates that are multivalent—that is, only antigens that have multiple antibody-binding sites epitopes. This allows for the formation of large antigen:antibody complexes.
References
Precipitin Wikipedia(Text) CC BY-SA