PRECIPITINS 245 



diagnosis (Malta fever), in most cases, however, 50 times 

 is the lowest limit. Evidently the greater the dilution, that 

 is, the higher the "titer," the more specific is the reaction. 



PRECIPITINS, 



Since agglutinins act on bacteria, probably through the 

 presence of substances protein in nature within the bacterial 

 cell, it is reasonable to expect that if these substances be 

 dissolved out of the cell, there would be some reaction 

 between their (colloidal) solution and the same serum. As a 

 matter of fact Kraus (1897) showed that broth cultures 

 freed from bacteria by porcelain filters do show a "precipi- 

 tate when mixed with the serum of an animal immunized 

 against the particular bacterium and that the reaction is 

 specific under proper conditions of dilution. It was not 

 long after Kraus's work until the experiments were tried 

 of "immunizing" an animal not against a bacterium or its 

 filtered culture, but against (colloidal) solutions of proteins, 

 such as white of egg, casein of milk, proteins of meat and 

 of blood serum, vegetable proteins, etc. It was ascertained 

 that in all these cases the animal's serum contains a sub- 

 stance which causes a precipitate with solutions of the pro- 

 tein used for immunization. The number of such precipi- 

 tating serums that have been made experimentally is very 

 large and it appears that protein from any source when 

 properly introduced into the blood or tissues of an animal 

 will cause the formation of a precipitating substance for its 

 solutions. This substance is known, technically as a 

 "precipitin." The protein used as antigen to stimulate its 

 formation, or some part of the protein molecule (hapto- 

 phore group), which acts as stimulus to the cell is spoken 

 of as a "precipitinogen," both terms after the analogy of 

 "agglutinin" and "agglutinogen." In fact the specific pre- 

 cipitation and agglutination are strictly analogous phenom- 

 ena. Precipitins act on proteins in (colloidal) solution and 

 cause them to settle out, agglutinins act on proteins within 

 cells which cells are in suspension in a fluid and cause the 

 cells to settle out. Ehrlich's theory of the formation of 



