466 PEACTICAL PHYSIOLOGY 



the supernatant fluid carefully, and test it and the sediment for the presence of 

 amboceptor and complement in the following manner : 



a. Add 0'2 c.c. of the supernatant fluid to 0'5 c.c. of the 5 per cent, suspension 

 of the washed erythrocy tes of the rabbit, and incubate at 40 for ten minutes. No 

 laking will probably be noted. Now add a few drops of inactivated immunised 

 dog's serum (see Experiment VII.), and incubate; laking will probably occur. The 

 conclusion is that the supernatant fluid contained complement, but no amboceptor. 

 If the experiment is unsuccessful it must be repeated again, using a smaller amount 

 of the supernatant fluid. 



b. Shake the sediment with a few c.c. of ice cold 0*9 per cent, sodium chloride 

 solution, and centrifugalise. Remove the supernatant fluid with a pipette and 

 place some of the washed sediment to a small test tube, and incubate ; only 

 slight laking, if any, will be noted in ten minutes. Now add a few drops of 

 rabbit serum or some of the supernatant fluid used in a, when laking will 

 become marked. The conclusion is that the amboceptors had combined with 

 the erythrocytes, but not with the complement. 



Precipitins. Not only cells and toxins, but proteins also, react in the manner 

 above described. The reaction manifests itself in the formation of a precipitate. 

 For example, the serum of one animal can cause the serum protein of another 

 animal to become precipitated, indicating that it contains some body which 

 changes the nature of the protein so as to render it insoluble. It is called a 

 precipitin, and, like anti-bodies in general, e.g. haemolysins, it seems greatly 

 increased in amount by inoculation. Thus, if rabbit serum be mixed with ox 

 serum the mixture remains clear, or only becomes slightly cloudy. If, however, 

 ox serum be injected intraperitoneally or intravenously into a rabbit, and the 

 inoculation repeated every three or four days for some weeks, then the rabbit 

 serum will, when mixed with ox serum, cause a marked precipitation of the 

 proteins. A precipitin has become developed by inoculation. Precipitins are 

 remarkably specific in nature, so that if a series of rabbits be inoculated as above 

 described, each with the blood serum of some different animal, there will be pro- 

 duced in the serum of each rabbit a precipitin which reacts only with the serum 

 of the same kind of animal whose blood was used for inoculation, or with that of 

 some closely allied species. 



In this way a variety of sera can be prepared (and preserved in sealed tubes), 

 which are peculiarly delicate and specific reagents for the detection of the proteins 

 used to produce precipitins in them. By an application of these facts it is possible 

 to distinguish from what animal a given blood serum (or blood solution) has been 

 obtained. Thus, if it be desired to ascertain whether a blood stain is of human 

 blood, it is dissolved in water and filtered till clear, and then mixed in a small 

 test tube with a few drops of the serum of a rabbit that has been inoculated for 

 some time with human blood. If a precipitate occurs the inference is that the 

 stain was of human blood. It is impossible to cause precipitin formation by 

 inoculation with blood from another animal of the same kind or from one 

 that is closely related. Thus, no precipitin will be developed in a rabbit serum 

 by inoculating the rabbit with guinea pig's serum. For the same reason, the 

 bloods of closely related animals can form precipitins that are common to them 

 all. Thus, if the blood of certain monkeys be inoculated in rabbits, the serum of 

 the rabbit will give the precipitin reaction not only with monkey's blood, but also 

 with that of man. 



