IMMUNITY AND PROTECTIVE INOCULATION 507 
practically no discomfort. It is, however, limited in its period of dura- 
tion. The most extensive use of passive immunity is in immunizing 
swine against hog cholera by the serum method and horses and peo- 
ple against tetanus by means of tetanus antitoxin. 
Explanation of acquired immunity. A number of ingenious expla- 
nations have been offered for acquired immunity. The only ones of 
the older theories that have withstood the test of their objectors are 
those of Metchnikoff, representing the cellular theory, and of Ehrlich, 
representing the humoral theory. Vaughan has advocated the pro- 
teolytic enzyme production as an explanation for bacterial immunity. 
The results of various investigations indicate that the factors involved 
in securing immunity are multiple in number and varied in character. 
As pointed out by Meltzer it is difficult to explain immunity as being 
due to one or even to a few anti-bacterial properties of the animal 
body. In the struggle against bacteria the defence of the body is 
carried on by the united action of each and every resisting influence. 
However, a number of theories have been proposed to explain the 
phenomenon. 
The exhaustion theory. This was suggested in 1880 by Pasteur, who thought that the 
microorganisms growing in the body used up some substance essential to their further 
existence and died leaving the body unsuited for future occupation. It applied to active 
immunity only. 
The retention theory. This was proposed by Wernich and Chauveau. It is based on 
the fact that bacteria elaborate some metabolic product that inhibits their further 
development and the future invasion in the tissues by the same species. This theory 
is ifustrated by the facts shown in the cultivation of bacteria in artificial media. 
The bacteria often die apparently from the accumulation of metabolic products long 
before the nutriment is exhausted. 
The phagocytosis theory. Metchnikoff has supposed that acquired immunity is 
brought about because of the action of the phagocytes upon the invading organisms. 
He has shown that in cases of infection with the Vibrio Metchnikovi, the phagocytes of 
unprotected animals do not take up the bacteria, but vaccinated animals do. It 
appears from the work that has been done on this subject that the phagocytes are 
active in proportion to the degree of immunity possessed by the individual. 
The humoral theory. This is based on the observations of Buchner, Nuttall and 
others that blood serum has the power of destroying a certain number of bacteria when 
they are placed in it. Nuttall showed in addition to this that the bacteriolytic power 
ceased if the blood was heated to 55° C. It is found, however, that the bacteriolytic 
serums occur only in cases where there is a high degree of forced immunity, their 
activity being in proportion to the degree of immunity obtained. An explanation for 
the action of the serums upon bacteria is given by Ehrlich in his lateral side-chain 
theory. 
Ehrlich’s side-chain theory. According to Ehrlich, in every living cell there must 
exist an active central body and a number of other chemical groups or side chains. 
