IMMUNITY, 109 
These protective substances are found also in other 
fluids of the body than in the blood; they occur, in- 
deed, in the substance of all cells to a greater or less 
extent. How much of this is simply in solution from 
the serum, and where the substances are tormed, is not 
definitely known. 
5. By the injection of the blood-serum of animals . 
which have previously passed through a specific disease 
or have been inoculated with the bacterial products. 
The first, probably, to think of the possibility of effect- 
ing this was Raynaud, who, in 1877, showed that the 
injection of large quantities of serum derived from a 
vaccinated calf into an animal prevented its successful 
vaccination. Héricourt, Richet, and others demon- 
strated the same thing for other diseases. The results 
obtained by Behring and Kitasato upon diphtheria and 
tetanus, where, indeed, the serum prevented the action 
of the poisons rather than the direct development of the 
bacteria, gave a still greater impetus to these investiga- 
tions. 
The immunity produced by these substances affects 
the entire body, as is only natural, since the blood into 
which they are absorbed is distributed everywhere. 
When the immunity is but slight, infection may take 
place in the more sensitive regions and still be im- 
possible in those tissues having more natural resistance. 
If the serum is injected into other animals or man the 
immunity is greatest immediately after absorption, and 
then declines, being rather quickly (in several weeks or 
months), almost entirely lost, so that repeated injections 
are required to maintain the immunity. This is dis- 
tinctly in contrast to the immunity acquired after the 
introduction of bacterial products, where the tissues 
