572 MICROBIOLOGY OF THE DISEASES OF MAN AND ANIMALS. 



occur. Again, it has been shown that only in rare instances animals 

 do produce precipitins for members of the same species. For example, 

 if an animal, such as a goat, is bled and the serum injected into 

 another goat, it is only in rare cases that the second goat will produce 

 an antibody which is capable of producing precipitation of the proteins 

 in the first goat's serum. Such precipitins are known as isoprecipitins 

 and occur only in a very small per cent of cases and with no regularity. 



The Phenomenon of Specific Inhibition. When precipitins are heated 

 to low temperatures (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 available precipitin- 

 ogen 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 serum 

 due to the injection of the antigenous substance which in this instance 

 is known as the precipitinogen, this precipitin, which is a definite anti- 

 body, 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 antianti- 

 body and is one of the few examples we have in immune reactions of such 

 substances. The antiantibody is the boundary for antibody formation. 



The Precipitinogen. As before stated, 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 destroyed 

 there results a body which is exactly analogous to the precipitoid produced 

 by heating the precipitin. Some bodies are known as the precipitoids of 

 the precipitinogen in distinction from the precipitoids of the precipitin. 

 These precipitoids retain their power to combine with precipitin, but no 

 precipitate results on such combination. 



The Precipitate. When precipitin and precipitinogen combine it 

 requires some little time before precipitation occurs. This is dependent 



