IMMUNITY. 171 



goose and hedgehog, and less markedly so of hogs. The crocodile 

 is insusceptible to both the tetanus bacillus and its toxin, but is sus- 

 ceptible to diphtheria toxin. Many birds, including the barnyard 

 fowl, are insusceptible to the tetanus toxin. The best explanation 

 that has been given to natural immunity against toxins is that offered 

 by Ehrlich's theory. This author holds that in order for a substance 

 to be toxic to the animal organism the latter must contain cells, the 

 molecules of which furnish side chains capable of forming chemical 

 compounds with the toxin.^ When the toxin is not capable of form- 

 ing a chemical combination with some constituent of the cell, it has 

 no poisonous action on that animal. Some bacterial toxins, such as 

 that of tetanus, form a destructive combination with the cells of the 

 central nervous system, while others possibly form similar compounds 

 with other tissues of the body. Therefore, according to Ehrlich, 

 natural immunity to toxins is due to the failure to form combinations 

 in the animal body which are destructive to some of its cellular 

 elements. 



The study of acquired immunity has furnished a rich field for 

 research to the bacteriologist. Early in his development man must 

 have observed that one attack of certain diseases gave to those who 

 recovered more or less permanent immunity to that disease. The 

 Chinese recognized this fact many centuries ago, so far as smallpox 

 is concerned, and this led to their practice of inoculation for this 

 disease. This custom probably spread from the Orient through 

 Tartar tribes, and, as is well known, was introduced into England 

 from Turkey. The next step that was made in the study of acquired 

 immunity was the discovery of vaccination for smallpox as practiced 

 by Jenner; but inasmuch as up to that time the specific bacteria 

 remained unknown, the philosophy of vaccination could not be ex- 

 plained. The scientific study of acquired immunity may be said to 

 date from the early investigations of Pasteur, who in 1880 discov- 

 ered that inoculations with non-virulent cultures of chicken cholera 

 gave immunity to subsequent inoculations with a virulent culture 

 of this microorganism. Indeed, as early as 1877 Pasteur observed 

 that susceptible animals inoculated with cultures of the anthrax 

 bacillus mixed with other bacteria did not acquire the disease. This 

 discovery, as we shall see later, has been amplified by others, espe- 

 cially by Emmerich. It may be well for us to proceed in the discus- 

 sion of acquired immunity by considering the different agents em- 

 ployed in securing this condition. They are as follows : (1) By 

 treatment with weakened cultures of the germ of the disease. (2) 

 By the employment of sterilized cultures of the specific microorgan- 

 ism or by the use of its toxins. (3) By treating the animal with 

 cultures of the specific organism mixed with other bacteria, or by 

 the employment of mixed cultures in which the specific microorgan- 

 ism is present. 



