IMMUNITY AND SUSCEPTIBILITY. 571 



substances of most bacterial cells and a large variety of other plant 

 and animal cells and constituents, such as blood serum, milk and grains. 

 They were first demonstrated in 1897 by Kraus, who noted that the 

 bouillon filtrates of cultures of B. typhosus, Bact. pestis, and Msp. comma 

 would cause precipitates when mixed with the antiserum taken from 

 cases of these diseases. The precipitin reaction is definite and specific. 

 The protein substance used in immunization is the only one which is 

 precipitated when the antiserum is added. To the protein substance 

 which produces the precipitins the name precipitinogen is applied. To 

 that substance in the blood serum and body fluids of the immunized 

 animal the name precipitin is applied. The combination between the 

 precipitinogen and the precipitin forms a new chemical substance known 

 as a precipitate. Precipitin may be formed in various parts of the body, 

 for example, in the parenchymatous cells of the organs and by the leu- 

 cocytes. Bact. diphtheria will not act as a precipitinogen and will not 

 produce precipitins. This is practically the only bacterium which will 

 not yield these antibodies. 



Normal Precipitins. Precipitins for alien blood serums have been 

 found in the organs and blood of seemingly normal animals. Normal 

 precipitins for bacterial proteins have not been demonstrated to a certainty. 



Mechanism of the Formation of Precipitins. The mechanism of the 

 formation of precipitins is similar to that of other antibodies. When 

 the precipitinogen is injected into the body of an animal, it combines 

 with certain of the body cells, occupying receptors which otherwise would 

 be used for the taking up of food products. As a result of this the cells 

 produce new receptors and the number of these more than compensates 

 for the ones already utilized. They are thrown off into the body fluids 

 and form the precipitins. It is supposed that the precipitinogen contains 

 haptophore receptors which combine with the haptophile receptors of 

 the cells. When these haptophile receptors are regenerated and pro- 

 duced in excess, as before stated, they are thrown off into the body fluids 

 and are really what we know as precipitins. Precipitins are produced 

 principally for widely different or heterologous substances or sera 

 (heteroprecipitins.) 



Autoprecipitins and Isoprecipitins. It has been demonstrated that 

 animals will not produce precipitins for their own protein substances. 

 For example, if an animal is bled and injected with his own blood serum 

 an antibody will not be produced. Therefore, autoprecipitins do not 



