238 A MANUAL OF BACTERIOLOGY 



microbes. If to such a filtrate in a test-tube a little of the corre- 

 sponding anti-seium be added by running in carefully, so that it 

 forms a layer at the bottom, an opalescent ring makes its appear- 

 ance at the line of junction of the two fluids. So also if an 

 animal be injected with milk, its serum, when added to milk of 

 the same kind as that with which it has been injected, causes 

 precipitation of the casein. This reaction is specific, and it is 

 thus possible to distinguish various milks from one another. 

 Similarly, anti-sera which produce precipitates, each with the 

 homologous substance, are obtained by the injection of peptone, 

 egg-albumen, blood-serum and other proteins. The latter 

 reaction has an important medico -legal application, for by means 

 of it the blood and flesh of different species of animals can be 

 distinguished. Thus the presence of horseflesh in sausages can 

 be detected. The method employed is to inject a rabbit intra- 

 peritoneally with four to six injections of defibrinated blood or 

 of blood -serum (or with a solution of the particular substance, 

 e.g. horseflesh), commencing with about 5 c.c. and increasing to 

 10 c.c. at intervals of a few days. After treatment the animal is 

 bled from an ear vein, and the serum is obtained. The blood 

 to be tested may be dried on filter-paper, pieces are then cut up, 

 a solution is made in 1-6 per cent, sodium chloride solution, and 

 to this specific serum is added. Tested in this way, human 

 blood anti-serum reacts i.e. forms a precipitate markedly 

 with human blood, less so with ape's blood, not at all with other 

 blood ; ox blood anti-serum reacts with ox blood, less so with 

 sheep, feebly with horse, hardly at all with dog. Mixtures of 

 bloods may also be tested. Precipitins are also formed naturally 

 in vivo. Thus the serum of a patient the subject of hydatid 

 disease gives a precipitate with hydatid fluid, and the reaction 

 may be used diagnostically. The production of the anti-body 

 seems to be due to the globulin constituent of the injected serum. 



It will thus be seen that the anti-bodies which result 

 from the injection info an animal of different substances 

 are extremely numerous and have varied properties, their 

 most notable characteristics being their extreme speci- 

 ficity and the extraordinary delicacy of the interactions 

 produced by them. It is important to note that these 



