APPENDIX A XXXIX 
RESEARCHES ON IMMUNITY. 
The researches into immunity stimulated by these discoveries 
and prosecuted by Ehrlich, Bordet and others have led to the develop- 
ment of a special field of research in Biochemistry, dealing with the 
reaction of living animals to living or dead colloidal substances intro- 
duced into them and forming the basis of sero-therapeutics and sero- 
diagnosis now so much employed in medicine. This may be illustrated 
by reference to the wonderful characteristics of our blood in its capa- 
bility of reacting against hostile invaders or indeed against foreign 
albumins in a dead condition introduced from without. Not only 
are antitoxins formed, but certain substances which have a character- 
istic or it may be a destructive effect on bacteria or foreign blood- 
corpuscles. In illustration of the former may be cited the so-called 
Widal reaction which consists in bringing together the blood of a 
typhoid-fever patient and a culture of typhoid bacilli. A peculiar 
agglutination of the bacilli is produced, due to the presence of some 
coagulating agent. This procedure has become very useful in the 
early diagnosis of typhoid fever. 
A phenomenon of an allied nature is now much employed in 
forensic practice for the recognition of human blood as distinguished 
from the blood of other mammals. Formerly the expert was merely 
able to judge from the size and form of the blood corpuscles that he 
was dealing with the blood of some mammal. The new method of 
diagnosis depends upon the discovery that the blood of one animal 
introduced into the body of another—that of man, for example, into 
the rabbit—produces a profound change in the blood of the latter. The 
altered blood serum (immune serum) produces a precipitate in a dilute 
solution of serum of the other which is not formed with serum from 
any other source. The precipitin and the precipitable substance are 
in such intimate relation to each other that this precipitin test is now 
relied on in criminal cases. At first it was not considered possible to 
distinguish satisfactorily between human blood and that of the higher 
apes; but a more sensitive test applicable to very minute quantities of 
blood has been discovered—the “fixation of complement” method. It 
appears that when precipitin and precipitable substance unite, an in- 
gredient of the blood-serum known as “complement” (apparently of 
the nature of a ferment because rendered inactive by exposition to a 
temperature of 50° C.) is fixed. The more or less exact correspondence 
_ of precipitin and precipitable substance can therefore be judged by 
the disappearance of free complement and consequently the more or 
less near blood-relationship of the forms examined. The method of 
procedure is as follows:—A definite quantity of immune serum (rabbit- 
anti-human, for example) is inactivated by heating to 56°C., as is 
