234 ISOPRECIPITIN 



we find them limited in all cases to the proteids. All the coagu- 

 lable proteids will give rise to the formation of precipitating sera. 

 As regards the formation of these substances by means of digested 

 proteids (peptone and albumoses), the facts are less certain. 

 Myers obtained a precipitin to Witte's peptone, but according to 

 Obermayer and Pick complete peptic digestion destroys both the 

 power of inducing the formation of antibodies and of being pre- 

 cipitated. The former is first to go, so that a partially digested 

 proteid solution which will no longer precipitate with an antiserum 

 will yet lead to the production of a precipitin when injected into a 

 suitable animal. The results with tryptic digestion appear to be 

 entirely similar. Complete destruction leads to loss of both 

 functions. It appears, therefore, that it is only the giant molecule 

 of proteid that can be agglutinated ; the smaller, diffusible 

 molecule of the products of proteolysis, like the molecules of 

 toxin, though they may, and probably do, unite with their anti- 

 bodies, do not manifest this combination by forming clumps. 

 Proteids which have been coagulated by heat have still the power 

 of forming a precipitin (which, of course, manifests its action on 

 solutions of the unaltered proteid) when injected. 



As regards the species of animal from which the precipitins are 

 to be prepared, it is natural to choose an animal as remote in the 

 zoological scale as possible. In the case of human sera or in- 

 different substances, such as solutions of albumin, etc., the rabbit 

 is usually chosen, since it is easily obtained and handled, and will 

 yield a very considerable amount of serum. Where antisera to 

 animals closely allied to rabbits are to be obtained, fowls or other 

 large birds are most suitable. There are, however, some observa- 

 tions which go to show that there are differences between individual 

 rabbits which are comparable in every respect to those between 

 the different red corpuscles of goats and other animals, as seen in 

 Ehrlich's experiments on the isohaemolysins. Thus Schiitze 

 injected the serum of rabbits into rabbits. In two cases out of 

 ten he obtained a precipitin which reacted with the serum injected. 

 This substance is called isoprecipitin. It may often be noted 

 that a precipitate falls when different samples of diphtheria anti- 

 toxin (in animals in which the earlier stages of the process of 

 immunization have been carried out by means of serum toxin) are 

 mixed together, and this is probably an analogous phenomenon. 

 These precipitins are, however, very feeble as compared with 

 those obtained by the injection of the serum of a certain species 



