198 PRECIPITINS 



typhoid bacillus, preferably one several weeks old, is filtered through 

 porcelain, in order to free it from the bacterial cells, then every third 

 or fourth day some of the filtrate is injected into a rabbit. Some days 

 after the last injection, serum obtained from this rabbit precipitates 

 the typhoid filtrate and that of no other bacterium. Other precipi- 

 tinogens are used in like manner with like result. The thing in the 

 bacterial culture or other fluid, which produces this effect, has never 

 been isolated. We only know that all precipitinogens are albuminous 

 substances, or derivatives of these, and that no fat, carbohydrate or 

 crystalloid has such action. Some precipitinogens contain only simple 

 proteins and these in small amounts, as is shown by the application 

 of tests for proteins. 



The Precipitins. The substance in the immune serum is known as 

 a precipitin. In the sera of certain animals, which have had no 

 treatment, there are natural precipitins. The serum of the untreated 

 ox, undiluted, precipitates many bacterial filtrates. Normal goat's 

 serum gives precipitates with the sera of rabbits and dogs. These 

 normal precipitins do not interfere with our tests because (1) they 

 are not specific and (2) they do not give the precipitates in high 

 dilutions. Not every animal produces precipitins. The best laboratory 

 animal for the production of precipitins is the rabbit. It is best in 

 case of blood tests not to select an animal closely related with that 

 from which the precipitinogen comes. The blood of one animal will 

 not develop a precipitin in another of the same species. The precipi- 

 tinogen may be administered intra-abdominally, subcutaneously or 

 intravenously. Administration by the mouth produces a precipitin 

 only when for some reason the precipitinogen is absorbed undigested. 

 This may result from overfeeding and easily happens in the young. 

 Puppies, kittens, and rabbits, up to ten days old, frequently absorb 

 unchanged proteins from the alimentary canal and this leads to the 

 production of precipitins. It has been found in a few instances that 

 the serum of infants precipitates cow's milk. This indicates that the 

 casein of the food has been absorbed undigested. It is possible that this 

 is a factor in the causation of cholera infantum. Especially is milk 

 partly changed by peptonizing bacteria likely to be absorbed without 

 digestion. 



