Precipitation Occurs When the Antigen is Soluble Instead of Cellular, Antisera, Optimal Proportions, Optimal Molar

Precipitation

	Precipitation Precipitation occurs when the antigen is soluble instead of cellular. Solutions of proteins or certain polysaccharides are used in precipitation tests with appropriate antisera. Precipitation occurs when antigen and antibody are mixed under correct conditions. The quantitative precipitation reaction is done in a series of test tubes.
A constant amount of antiserum and decreasing amounts of antigen are added to the tubes in a row. The tubes are then incubated. At a certain tube haziness appears, followed by flocculation and precipitation. The amount of precipitation increases along the row, reaching a maximum and then falling off with the lower antigen concentration. The tubes where most precipitate appears contain the optimal proportions of antigen and antibody.
The range over which precipitation occurs is referred to as the equivalence zone. In the series shown in the figure, the tubes to the left of the equivalence Zone represent the zone of antibody excess. Precipitation depends upon the formation of large insoluble aggregates of lattices. Precipitation occurs when aggregates grow to a critical size, at which they lose their solubility.
The size of aggregates formed is dependent on the ratio of antigen 10 antibody in the mixture. The tube of optimal proportions represents the ratio at which the aggregate or latice formation occurs optimally. The optimal molar ratio between antigen and antibody depends upon the size of the antigen and antibody, and the number of antigenic determinants it possesses. With excess of either reagent-antigen or antibody complexes form in which all combining sites on only, one reagent are occupied. These complexes are more or less soluble.

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Precipitation