ACQUIRED IMMUNITY 173 



immunity was illustrated by the ancient, obsolete, and discarded prac- 

 tice of smallpox inoculation, by which healthy persons were inoculated 

 with the virus of a mild case of smallpox, at a time when no epidemics 

 existed and the person was in good general health and able to secure 

 proper attention from the outset. 



This process of immunization is used much more extensively in 

 veterinary practice, where an occasional untoward or fatal result is of 

 comparatively little importance if by its means an outbreak can be con- 

 trolled or the great majority of the animals saved. As a rule, an at- 

 tempt is made to render the induced disease as mild as possible by (a) 

 Using a small amount of infective material; (6) by inoculating it through 

 an unusual avenue; or (c) by inoculating it at a time when the animals 

 are naturally less susceptible, or (d) by a combination of these methods. 

 For example, Texas cattle fever, which is due to a protozoan (Piro- 

 plasma bigeminum) conveyed by the bites of infected ticks, may be 

 combated by exposing calves while still milk fed to the bites of a few 

 infected ticks. Another method consists in injecting a small amount of 

 blood from an infected animal directly into the jugular vein. The ob- 

 ject is to induce a mild attack of the disease. Occasionally a severe or 

 fatal reaction occurs, but the number of these untoward results is much 

 lower than the mortality among untreated animals. 



(3) Active immunity may also be gained by vaccination, i. e. } by in- 

 oculation with a virus or microparasite or its products, modified and 

 attenuated by passage through a lower animal (Jennerian vaccination) 

 or by various other means, as age, unfavorable cultural conditions, heat, 

 germicides, etc. (Pasteurian vaccination or bacterination) . These subjects 

 are considered more fully in the chapter on Active Immunization. 



Active immunity, whether induced accidentally or artificially, may 

 be antitoxic, as after recovery from diphtheria or as the result of active 

 immunization with diphtheria toxin, as by von Behring's method; or 

 antibacterial, as the immunity following typhoid fever or induced by 

 typhoid vaccination, and largely dependent upon the presence of bac- 

 teriolysins in the circulating fluids. 



During the process of active immunization an animal not infre- 

 quently fails to react to relatively large doses of toxin, and at the same 

 time the quantity of antibody in the body-fluid may decrease. This 

 phenomenon has been explained as being due to atrophy of the receptors 

 of the body-cells (receptoric atrophy), whereby the toxin fails to exert 

 its deleterious influence because it fails to unite with the body-cells. 

 It is curious, however, that the toxin is innocuous when present in a 



