488 IMMUNITY 



especially to agglutination, that some reference may be made to it. 

 When the serum of an animal is injected in repeated doses into another 

 animal of different species, after the type of an immunisation, there 

 appears in the serum of the animal treated a substance called precipitin, 

 which causes a cloudiness or precipitate when added to the serum used. 

 This precipitate results from the union of the precipitin in the anti-serum 

 with a body in the homologous serum, the latter being known as the 

 precipitinogen. (In the case of rabbits doses of 3 to 4 c.c. of the serum 

 may be injected intraperitoneally at intervals of four to five days, a 

 precipitin usually appearing at the end of about three weeks.) The 

 reaction, which is a very delicate one, is conveniently observed by adding 

 a given amount of the anti-serum, say '05 c.c., to varying amounts of 

 the homologous serum '1, '01, etc. c.c., in a series of small test tubes, 

 the volume being then made up with salt solution to 1 c.c. In this way 

 a definite reaction may be observed with '001 c.c. of the serum or even 

 less. The reaction is specific in the sense explained above. It is always 

 most marked towards the serum of the species used in the immunisation ; 

 but while this is so, there may also be a slight reaction towards animals of 

 allied species. An anti-human serum, for example, gives the maximum 

 reaction with human serum, but also a slight reaction with the serum 

 of monkeys, especially of anthropoid apes ; it, however, gives no reaction 

 with the serum of other animals. The precipitin test has thus come to 

 be employed as a means of differentiating human from other bloods. 

 Another interesting phenomenon is what is known as the "deviation of 

 complement," which is produced by the combination of the two sub- 

 stances in the serum and anti-serum respectively. If mixtures be made 

 according to the above method, and then a small quantity of complement, 

 say fresh guinea-pig serum, be added, it will be found that the comple- 

 ment becomes absorbed, as may be shown by subsequently adding a test 

 amount of sensitised red blood corpuscles. This deviation phenomenon 

 is even a more delicate reaction than the precipitin test, it being often 

 possible to demonstrate by its use from a tenth to a hundredth of the 

 smallest amount of serum which will give a perceptible precipitate ; it 

 also is specific within the same limits. 1 



Therapeutic Effects of Anti-Sera. As will have been 

 gathered, the chief human diseases treated by anti-sera are 

 diphtheria, tetanus, streptococcus infection, pneumonia, plague, 

 and snake bite. Of the results of such treatment most is known 

 in the case of diphtheria. Here a very great diminution in the 

 mortality has resulted. The diphtheria antitoxin came into 

 general use about October 1894, and the statistics published by 

 Behring towards the end of 1895 indicated results which have 

 since been confirmed. In the Berlin Hospitals the average 

 mortality for the years 1891-93 was 36*1 per cent, in 1894 it 

 was 21 '1 per cent, and in January-July 1895, 14*9 per cent. The 

 objection that in some epidemics a very mild type of disease 

 prevails is met by the fact that similar diminutions of mortality 



1 For an account of precipitins, vide Nuttall, " Blood Immunity and 

 Relationships," Cambridge 1904 ; and of complement deviation, Muir and 

 Martin, Journ. of Hyg. vi., 1906, p. 265. 



