ANTITOXIC IMMUNITY. 175 



The use of blood serum of artificially immunized animals for this purpose is of far 

 greater practical importance than either of the methods 1 or 2. 



In passive immunity the protective material is furnished ready made, 

 the effect is secured at once and is temporary. 



Antitoxic Immunity. 



It appears from a study of the results of artificial immunization, as 

 well as of the immunity which is involved in the natural recovery from 

 infectious disease, that one of the ways in which immunity is secured is 

 by the formation of substances in the body fluids which in some way 

 neutralize or suspend the action of toxins antitoxic immunity. 



Thus in diphtheria and tetanus the antitoxic substances are largely 

 developed, while in many other infectious diseases, such as cholera and 

 typhoid fever, it appears to be, in part at least, through the bacterio- 

 lytic substances that protection is secured. While in many instances 

 both of these types of protective substances may be formed during im-' 

 munization, one or the other is usually preponderant. ' 



The antitoxic substances are most closely related to the globulins, 

 but beyond this their chemical nature has not been definitely ascertained. 2 



The effect of antitoxin in the blood in rendering harmless the toxic 

 substances which they form apparently is accomplished, not by the de- 

 struction of the toxins, but by a chemical union of toxin with antitoxin, 

 whereby the former is deprived of its capacity to injure cells. 



The knowledge of this antitoxic immunizing action of specially en- 

 dowed blood serum has been most fully developed in diphtheria and 

 tetanus. 



During the growth of the diphtheria bacillus in nutrient broth a toxic 

 substance is developed which mingles with the broth. This is called 

 diphtheria toxin, and subcutaneous injections of this toxin in animals 

 guinea-pigs, for example prove fatal, in appropriate dosage, with 

 symptoms and lesions similar to those caused by inoculation with the 

 living germ. It has been found that by repeated injections of the diph- 

 theria toxins in susceptible animals, at first with small, then with gradu- 

 ally increasing doses, the animal may at length become so insusceptible 

 to the action of the poison that many times the usually fatal dose is 

 borne without sensible reaction. Similar immunity can be conferred in 

 certain animals by the use of the living cultures of the diphtheria bacil- 

 lus either fully virulent or with reduced virulence (see p. 174), adminis- 

 tered at first in small doses which are gradually increased. 



In whichever way immunity be conferred, it has been found that the 



1 The damage or destruction of bacteria by anti-bacterial, bactericidal, or bacteriolytic 

 substances naturally limits or prevents the formation of toxic materials by the micro- 

 organisms, although the cells of the body may be very susceptible to the action of these. 

 So that the absence of evidence of the establishment of antitoxic immunity in an infec- 

 tious disease may mean, not that the body is incapable of this, but only that no oppor- 

 tunity is offered for its development. 



2 See Hiss and Atkinson, Journal of Experimental Medicine, vol. v., p. 47, 1900. 



