152 CHEMISTRY OF IMMUNITY AGAINST BACTERIA 



(Neisser and Friedmann l ) which form an electrically amphoteric 

 colloidal suspension, so that the ions of electrolytes or the 

 electric currents, by discharging them unequally, cause precipita- 

 tion. Physico-chemical researches, however, have yet failed 

 to explain the specific character of the agglutinins for specific 

 bacteria. 



PRECIPITINS 2 



If to the filtrate from a bacterial culture we add in proper 

 proportions the serum of an animal immunized against the same 

 variety of bacteria, or against their cell contents, a precipitate 

 will soon form. This reaction is specific in that it is produced 

 to a much less degree, or not at all, with cultures of bacteria 

 different from the variety used in immunization. The precipi- 

 tated substances seem to consist of the soluble proteid con- 

 stituents of the filtrate derived from the bacterial cells. 



This reaction seems to be of little significance as a means of 

 defense against bacterial invasion ; but the discovery that all 

 forms of proteids when injected into animals may cause the 

 appearance of specific precipitating substances in the serum of 

 the animals, has led to most important applications of the pre- 

 cipitation reaction. Apparently every variety of proteid is in 

 a certain sense poisonous to animals that do not normally have 

 it in their blood or tissues, and its injection leads to a reaction 

 on the part of the animal, which reaction is shown by symptoms 

 of sickness and the appearance of the specific precipitating 

 substances in the serum. It is the sharp limits of specificity 

 that render the precipitin reaction of such importance, for if we 

 immunize an animal with globulin from the blood of a horse, its 

 serum will precipitate only globulin from horse blood, and not 

 globulin from the blood of a dog, or man, or any other animal. 

 Similarly, if the immunization is with cow's milk, the serum will 

 precipitate only cow's milk and not the milk of the goat or 

 mare. These serum reactions are of importance to the physio- 

 logical chemist, therefore, since they furnish a means of distin- 

 guishing between closely related forms of proteids, more delicate 

 by far than any known chemical reagent. They also prove that 

 there are essential chemical differences between the proteids of 



1 Loc. cit. ; see also Girard-Mangin and Henri, Compt, Kend. Soc. Biol.. 1904, 

 vol. 56 ; and Zangger, Cent. f. Bakt. (ref.), 1905 (36), 225. 



2 For complete bibliography of the subject of " Precipitins" see the resume 

 by Michaelis, Biochemisches " Centralblatt, 1905 (3), 693; and Zeit. f. klin. 

 Med., 1905 (56), 409 ; Blum, Cent. allg. Path., 1906 (17), 81 ; Pfeiffer, Arch. f. 

 Kriminalanthropol., 1906, Bd. 22. For methods and earlier literature see 

 Nuttall, Jour, of Hygiene, 1901 (1), 367. 



