Precipitation and agglutination.

Immune precipitation is the formation of insoluble complexes as a result of the specific interactions between antigen molecules and the corresponding antibody molecules, both in aqueous solution. Usually, the largest amounts of precipitate are obtained when soluble antigens and antibodies are present in approximately equal concentrations. Therefore, to determine the concentration, or just the presence of antibody, precipitation requires considerable amounts of antigen; the method usually does not allow the detection of less than microgram quantities of antibody; that is, it is about loo0 times less sensitive than agglutination. Contrary to agglutination, divalent IgG (and not decavalent IgM) is the immunoglobulin with the strongest precipitating power. A major breakthrough that caused the diversification of immune precipitation into a variety of different and powerful analytical methods was the development of precipitation in gels. This approach gave rise to methods permitting, for example, the distinction of small differences between antigenic sites, and the characterization of 100 or more different blood serum proteins.