PRAECIPITINS 465 



PR^CIPITINS 



Not only toxins and living cells, but also proteids, can, 

 when injected into an animal, lead to the production of specific 

 bodies with which they react. In this case the reaction takes- 

 the form of a precipitation, hence these bodies are called 

 Praecipitins. For example, the normal blood serum of a rabbit 

 mixes with the normal blood serum of a horse without any pre- 

 cipitate forming ; if, however, horse's serum be frequently inocu- 

 lated in small dosage into a rabbit, the serum of the latter will, 

 after some time, produce a precipitate when mixed with horse's 

 serum (Tsistowitsch 7 ). It may, in fact, be stated as a law that 

 the repeated inoculation into an animal of a proteid foreign to its 

 tissues leads to the production, in the blood serum, of a praecipitin 

 acting only on the proteid used for inoculation. In such experi- 

 ments with blood serum there will, of course, be produced, besides- 

 the praecipitins, various other anti-bodies (anti-complement, anti- 

 agglutinins, &c.). The mechanism involved in the production of 

 praecipitins is probably identical with that involved in the pro* 

 duction of agglutinins i.e. receptors of the second class are re- 

 generated in over excess as a result of similar receptors having been 

 fitted by the injected proteid. 



A large number of praecipitins have been described, and it 

 may be of interest to mention a few of the more typical of these. 

 If cow's milk be injected into a rabbit, the rabbit's serum will 

 precipitate the caseinogen of cow's milk (lactoserum) ; albuminuric 

 human urine produces in the serum of an injected animal a prae- 

 cipitin for the proteid of albuminuric urines; vegetable proteid, 

 a serum precipitating vegetable proteid ; egg albumin, a serum 

 precipitating egg albumin ; peptone, one precipitating peptone, 

 and so forth. It is of importance to note here, however, that 

 praecipitins produced by the injection of the blood serum of one 

 animal into another react with the serum of all closely allied 

 animals. Thus if human serum be inoculated into a rabbit, the 

 praecipitin thereby produced in the rabbit will precipitate the 

 proteid, not only of human serum, but also of that of the higher 

 apes. Similarly, the serum of a rabbit, inoculated with the serum 

 of a hen, can precipitate the proteids in the serum of a pigeon. 

 In the same way no praecipitin is produced by injecting the 

 blood serum of an animal into another animal of the same genus. 



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