SPECIAL BACTERIOLOGY 
171 
dueed on a large scale, by injecting horses with gradually 
increasing doses of toxin, until according to Weigert’s 
law, many more antibodies (antitoxin) are produced than 
are really necessary to protect the horse. The blood 
serum is removed, standardized (that is, its strength is 
determined), and it is then ready for use. It is evi¬ 
dent that immunity conferred by the injection of anti¬ 
toxin serum is acquired artificial, passive immunity. 
VII. Bacteriologic Diagnosis 
Bacteriologic diagnosis rests on peculiar beaded ap¬ 
pearance of a bacillus, which is often club-shaped, Gram- 
Fig. 33.—Pseudodiphtheria bacilli. (B. hofmanni.) (Park and Williams— 
Pathogenic Bacteria and Protozoa .) 
negative; cultural appearance on Loeffler’s beef blood 
serum is diagnostic: small, round, glistening, grayish 
white colonies. 
VIII. Immunity Treatment 
Together with the smallpox vaccination, the preven¬ 
tion and treatment of diphtheria with the antitoxin serum 
(initiated by Behring, Roux and Yersin) is, without 
doubt, the greatest accomplishment achieved by medicine. 
No one today realizes what diphtheria meant in former 
