IMMUNITY AND SUSCEPTIBILITY 719 



that animals will not produce precipitins for their own protein sub- 

 stances. For example, if an animal is bled and injected with its own 

 blood serum an antibody will not be produced. Therefore, autofre- 

 cipitins do not occur. Again it has been shown that only in rare 

 instances do animals produce precipitins for members of the same spe- 

 cies. For example, if an animal, such as a goat, is bled and the blood 

 serum injected into another goat, it is only in rare cases that the second 

 goat wUl produce an antibody which is capable of producing precipi- 

 tation of the proteins in the first goat's blood serum. Such precipitins 

 are known as isoprecifiHns and occur only in a very small per cent of 

 cases and with no regularity. 



The Phenomena of Specific Inhibition. — When precipitins are heated 

 to low temperature (50° to 60°) or are subjected to the action of 

 light or certain chemicals, their power to produce a precipitate when 

 combined with a precipitinogen is destroyed. The precipitin which 

 has been heated becomes a precipitoid similar to an agglutinoid or a 

 toxoid. Their ability to combine with the precipitinogen still remains. 

 It is possible, therefore, for precipitoids to combine with all the avail- 

 able precipitinogen so that when fresh precipitin is added no precipitate 

 will occur. This is known as specific inhibition and sometimes leads 

 to very confusing findings in the study of these immune bodies. 



Antiprecipitins. — When an animal has produced a precipitin in its 

 blood serum due to the injection of the antigenous substance which in 

 this instance is known as the precipitinogen, this precipitin, which is 

 a definite antibody, may be used for the immunization of another 

 animal and an antiprecipitin produced; that is, a body which will 

 combine with the precipitin in such a way as to prevent precipitation 

 when this substance is combined with the precipitinogen. This 

 is then, in fact, an antiantibody and is practically the only example we 

 have in immune reactions of such a substance. The antiantibody is 

 the limit for antibody formation. 



The Precipitinogen. — As before stg,ted, the precipitinogen is any 

 protein substance which will cause the formation of precipitins. Cer- 

 tain of the precipitinogens are composed of two groups, one which is 

 thermostable and another which is thermolabile. Therefore, when 

 these precipitinogens are heated and this thermolabile substance de- 

 stroyed there results a substance which is exactly similar to the precipi- 

 toid produced by heating the precipitin. Such bodies are known as 



