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 iu 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 



