PRECIP1TIXS 153 



different species of animals, even when by all other methods 

 these proteids seem to be practically identical ; e. </., lactalbumin 

 of cow's milk is in some respect different from lactalbumin of 

 goat's milk since it produces a different precipitin. To the 

 physiologist they indicate the method adopted by the body to 

 guard itself against invasion by foreign proteids introduced in 

 the food, and show the importance of the complicated digestive 

 and assimilative mechanism of the alimentary tract in securing 

 complete destruction of the specific characters of all proteid foods 

 before they enter the blood. Clinically they offer a means of 

 detecting abnormal permeability of the walls of the digestive 

 tract, and possibly a means of determining the source of proteids 

 found in the urine. Medicolegally they offer an accurate method 

 of determining the origin of blood and serum stains, no matter 

 how old the stain may be; thus Hansemann l found that 

 material obtained from a mummy 5000 years old gave the 

 precipitin reaction. 



Production of Precipitins. For the production of the 

 precipitation reaction it is necessary to have in the substance 

 used for immunization a certain group, the precipitogen, which, 

 when injected gives rise to production of precipitin by the 

 animal. 2 Apparently any proteid may act as a precipitogen 

 if injected into the proper animal, but it must be a foreign 

 proteid; rabbit serum will not produce precipitins if injected 

 into a rabbit, 3 probably because it is normally present in the 

 blood of the rabbit and therefore does not stimulate any 

 reaction. In general the more foreign the proteid, the greater the 

 amount of precipitin ; closely related animals, e. g., rabbit and 

 guinea-pig, produce little precipitin for one another's proteids. 

 This indicates distinctly that difference in species depends upon 

 or is associated with difference in chemical composition of the 

 proteids. Only proteids can produce precipitins ; when split 

 to the peptone stage they lose this property. No precipitins 

 can be secured against the other food-stuffs ; i. e., carbohydrates 

 and fats. Possibly precipitins can be produced for closely 

 related substances with molecules approximating in size the 

 proteid molecule, e. g., certain substances present in proteid-free 

 filtrates of bacterial cultures. 



Since precipitation of colloids is accompanied by or dependent 



1 Munch, med. Woch., 1904 (30), 572. 



2 Krausand Schiffmann (Wien. klin, Woch., 1905 (18), 1033) believe that 

 precipitins as well as agglutinins are formed in the circulating blood > not in 

 the organs. 



3 Rarely a slight reaction against homologous proteids has been obtained 

 (iso-precipitins). 



