250 hnmunity in Some of its Biochemicol Aspects [Jan. 



The means of protection we have just recited are all general in 

 their action, that is, not directed specifically against only one partic- 

 ular infection, Let us now return to a consideration of the specific 

 acquired immunity already mentioned. 



Behring's DISCOVERY OF ANTITOXIN, Most of our knowlcdgc 

 concerning specific acquired immunity dates from Behring's discov- 

 ery of the antitoxins of diphtheria and tetanus, in 1890. 



Behring found that when an animal is injected with gradually 

 increasing doses of toxin, e. g., with diphtheria toxin, it is able, after 

 a time, to withstand doses of the poison sufficient to kill hundreds 

 of animals not so treated. He found that the blood serum of the 

 treated animals contained something which neutralized the diph- 

 theria poison, and rendered it harmless. This something he called 

 an antitoxin. Investigation showed that the antitoxin was strictly 

 specific, the antitoxin for diphtheria neutralized only the toxin of 

 diphtheria, the antitoxin for tetanus, only that of tetanus. 



Bacteriolysins and hemolysins (cytolysins). Another 

 important advance was made in 1894 when Pfeiffer showed that, 

 just as an animal injected with gradually increasing doses of toxin 

 produces an antitoxin in its blood, so also, when injected with bac- 

 teria (cholera bacilli), it produces substances which kill and dissolve 

 the injected micröorganisms. We have already said that fresh 

 blood serum is able to kill a considerable number of bacteria, and 

 that this probably constitutes one of the defenses of the body against 

 bacterial Invasion. When the animal is injected with gradually in- 

 creasing amounts of bacteria, however, this destructive power in- 

 creases very greatly, but only for the particular kind of bacterium 

 used for injection. In other words, the action is strictly specific. 

 If an animal is injected with cholera bacilli, the serum will, after a 

 time, even in very small doses kill enormous numbers of cholera 

 bacilli ; tested against typhoid bacilli, or on other bacteria, its de- 

 structive effect is merely that of normal serum from an untreated 

 animal. When the action of the serum is studied under the micro- 

 scope, it is seen that the bacteria are actually broken up and dis- 

 solved. Hence such a serum is spoken of as a " bactcriolysin." 

 Since the bacteria are also killed by this action, we also use the term 

 " bactericidal " in speaking of such a serum. 



