244 ILLINOIS ACADEMY OF SCIENCE 
tinins, lysins and opsonins. There is no doubt that the effi- 
ciency of bacterins depends upon their power to stimulate the 
formation of opsonins. 
The serum or its modified and purified preparation, the 
globulin solution, is the older and best known of these two 
classes, i.e., bacterines and serums. The first successful serum, 
the diphtheria anti-toxin, was discovered by Behring and Kit- 
asato in 1890. Anti-toxins are produced within the body 
of some animal, the horse being used in most cases. Such 
an animal is given injection of the toxins, rarely of dead 
bodies, of a specific disease producing micro-organisms in in- 
creasing quantities, until the point of tolerance or maximum 
resistance has been reached. As a result, anti-toxic sub- 
stances are produced by these animal cells and they appear in 
the serum. At the proper time the animal is bled and after 
various stages of purification and concentration, the blood 
serum is put up in suitable containers for administration to 
the human patient. 
This serum contains anti-bodies or immune substances 
which will directly combat the specific infection. By the in- 
jection of a serum the physician produces passive immunity. 
In order that a serum be of greatest therapeutic value it must 
be of a standard specified strength so that the size of the dose 
administered may be regulated. This process of regulating 
serums and bacterines is termed standardization, and in the 
case of the anti-toxin serum it is done in terms of "anti-toxin 
units." To test the anti-toxic value guinea pigs of about 
250 grams are used. These guinea pigs and the parents 
of these guinea pigs should never before have been used in the 
testing of anti-toxin. An anti-toxic unit is to be understood 
by its effect only. 
A unit is capable of neutralizing an amount of toxin, or 
bacterial poison; that is, in turn, measurable by its fatal effect 
on guinea pigs in the presence of a standard immunity unit 
furnished by the United States Government. The immunity 
unit is mixed with the toxin and administered to guinea pigs. 
Sufficient toxin must be used to kill the guinea pig notwith- 
standing the protection afforded by the immunity unit. One 
anti-toxic unit will just save the life of the guinea pig when 
injected together with the toxin dose above mentioned. 
