28 Predisposition and Immunity. 



protoplasm, the production of antibodies will be again induced, 

 receptors being again formed. This form of immunity, produced 

 through cellular activity following the introduction of the infec- 

 tious agents, is known as "'active immunization." The discovery 

 of the existence and production of specific antibodies in the serum, 

 for which we are mainly indebted to the studies of Behring upon 

 tetanus- and diphtheria-immunity, has led to extremely im- 

 portant methods of prophylactic and therapeutic inoculation against 

 a number of the infectious diseases. This becomes possible from 

 the fact that the blood* serum of an animal which has survived a 

 certain infection and is highly immunized against it can be em- 

 ployed by injection to produce in a second animal similar im- 

 munity against the same disease. The production of the pro- 

 tective material in the blood depends upon the introduction into 

 the experiment animals, either intravenously or subcutaneously, of 

 gradually increasing amounts of the infectious material against 

 which immunization is sought. The animals in this way become 

 more and more highly immunized and their blood serum becomes 

 correspondingly rich in antitoxine. By such a system of pro- 

 gressive inoculation it is possible to attain a degree of concentra- 

 tion in which a thimbleful of the serum contains more immuniz- 

 ing material than would exist in all the blood of the body after 

 one single attack of the disease. The ability to produce such 

 materials exists in the cells of all kinds of animals, both those 

 susceptible to the disease in question and those naturally resistant 

 to it. For example, by injecting the bacilli of swine-erysipelas, 

 which in natural conditions are pathogenic only to the bristled 

 kine, it is possible to induce the reaction also in sheep, goats, 

 cattle and horses and produce in their blood immunizing substances ; 

 and then, after withdrawal of the blood from animals thus pre- 

 pared, to obtain from it a prophylactic and curative serum in suffi- 

 cient quantity for use in hogs. The production of active im- 

 munity in an individual requires some time, because the body cells 

 must first form the protective substances ; in other words, must 

 first pass through the period of the disease, perhaps four to six 

 weeks in duration. \A'hen, however, the prepared serum is intro- 

 duced there is no such demand placed on the body cells ; the pro- 

 tective substances, already formed, produced from the animal 

 supplying the serum are introduced into the second animal. An 

 immunity thus obtained is spoken of as "passive immunity." It 

 comes on immediately or in a few hours after the inoculation. 



