IMMUNITY. 115 
fourth of that furnished by others. The antitoxin pro- 
duced by a certain number of fatal doses of toxin will 
neutralize many thousand times that amount. 
Diphtheria and tetanus antitoxin are measured by the 
protective power of the serum in which they are in 
solution—that is, the amount of serum required to pro- 
tect susceptible animals from a certain number of fatal 
doses. In diphtheria this is measured in units; in 
tetanus usually by the proportion which exists between 
the amount of serum used and the animal’s weight. For 
detailed information, see under Diphtheria and Tetanus. 
Antitoxins are absorbed to a very slight extent only 
when taken by the mouth—certainly less than 5 per 
cent. They must, therefore, be introduced subcuta- 
neously or intravenously to enter the body. The anti- 
toxic serum does not act against the bacteria directly, 
but by neutralizing.their poison, it prevents them from 
acting as irritants to the cells, and so the soil for the 
growth of the bacteria becomes unsuitable, and they 
cease to develop. The diphtheria bacilli grow perfectly 
well in their antitoxic serum. 
The Elimination of the Bacteria and Their Products. 
This takes place by the direct separation and removal 
of the bacteria where there is access to the outside, 
such as exists in the mucous membranes of the respi- 
ratory, digestive, and urinary tracts, and from the 
cutaneous surfaces, etc. The elimination of the bac- 
teria and their products is almost a necessity where 
there has been any great accumulation, if healing 
occurs. When the bacteria have penetrated deeply 
into the tissues, and continue steadily advancing, the 
elimination from the surface is of little curative value, 
as the number thrown off is so small in comparison 
