PRECIPITINS 189 



may be practically inagglutinable even by active scrum, but after pro- 

 longed cultivation on media they may or may not develop agglutina- 

 bility. This phenomenon has not yet been satisfactorily explained, 

 but it may depend on an active immunity of the bacteria against the 

 agglutinins. Such bacteria injected into rabbits produce antisera 

 that will agglutiiuitc ordinary agglutinablc strains, but not themselves; 

 hence they do not lack agglutinogens. They give normal complement 

 fixation reactions, and hence do not lack receptors, and they agglu- 

 tinate with acids and chemicals much tlie same as ordinary agglutinable 

 strains. ^^'"^ 



PRECIPITINS *■' 



If to a solution containing proteins we add in proper proportions 

 the serum of an animal inmiunized against the same protein, a pre- 

 cipitate will soon form. AVliile not absolutely specific, the quantitative 

 specificity of the precipitin reaction is sufficiently characteristic to be 

 of great value in biological, bacteriological, and medicolegal work, 

 and it is of importance to the physiological chemist, since it furnishes 

 a means of distinguishing between closely related forms of proteins, 

 more delicate by far than any known chemical reagent. The serum 

 reactions also prove that there are sometimes essential differences be- 

 tween the proteins of different species of animals, even when by all 

 other methods these proteins seem to be practically identical ; e. g., 

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

 bumin of goat's milk since it produces a different precipitin. Medi- 

 colegally they offer an accvirate method of determining the origin of 

 blood and serum stains, no matter how old the stain may be ; thus 

 Hansemann *** found that material obtained from a mummy 5000 

 years old gave the precipitin reaction.-" 



Production of Precipitins. — For the production of the precipita- 

 tion reaction it is necessary to have in the substance used for immu- 

 nization a certain group, the precipitinogen, which when injected gives 

 rise to production of precipitin by the animal. Apparently almost 

 any protein may act as a precipitinogen if injected into the proper ani- 

 mal, but it must he a foreign protein; rabbit serum will not produce 

 precipitins if injected into a rabbit,^^ probably because it is normally 

 present in the blood of the rabbit and therefore does not stimulate 

 any reaction; but certain chemical alterations in the proteins of an 

 animal, such as heating, iodizing, or partial digestion, may render them 



84a Mcintosh and McQueen, Jour. Hyg., 1914 (1.3), 409. 



85 For complete bibliocraphy of the subject of "Precipitins" see the resume by 

 Michaelis, Oppenheimor's Handb. d. Biochemie. 1009. II (1), ,5.52; Kraus. Kolje 

 and Wassermann's Handb.. 1913, II; Uhlenhuili and Stefl'enhairen. ihid.. III. 2.i7: 

 Zinsser. "Infection and Eesi stance."' 



sGMiinch. med. Woch.. 1904 (.30), 572. 



87 Not corroborated by Scliniidt. Zeit. allor. Pliysiol., 1907 (7), 369. 



88 Rarely a slight reaction against homologous proteins has been obtained {iso- 

 precipitins ) . 



