128 THE DIFFERENT TYPES OF IMMUNITY 



also, and by experimentation in this direction he laved the founda- 

 tion of our modern vaccine therapy, which finds its most important 

 expression, so far as human pathology is concerned, in the curative 

 treatment of rabies, and in the prophylactic vaccination against 

 typhoid fever. In the laboratory it has further led to the recognition 

 of the fact that even though immunity cannot be produced against 

 all pathogenic organisms by vaccination, it is at least possible to 

 bring about a marked increase in resistance, and by applying this 

 principle to other infectious diseases to which man is subject, a 

 radical advance in the rational treatment of these maladies has 

 been achieved (see section on Vaccine Therapy). 



Antitoxic Immunity. In a previous chapter we have seen that some* 

 pathogenic organisms injure the host into which they have been 

 introduced through the products of their metabolism or degenera- 

 tion, in so far as these are of toxic character, while their infectious- 

 ness may be of a very low order. Others produce a harmful effect 

 directly in consequence of their high grade of infectiousness, even 

 though they do not give rise to toxic products, while in still other 

 cases we see both factors variously combined. Evidently, then, 

 the existence of a natural immunity, or of immunity brought about 

 as a consequence of infection, may manifest itself either as a resist- 

 ance of variable degree against the development of microorganisms 

 in the body of the infected animal, or as a resistance against bacterial 

 toxins, endotoxins, aggressins, etc., or it may be directed against 

 both. It is hence appropriate to speak of antitoxic immunity on 

 the one hand, and antibacterial immunity on the other. 



As an example of natural antitoxic immunity we may mention 

 the natural resistance which the alligator offers to the action of 

 tetanus toxin, while the steadily increasing resistance to diphtheria 

 toxin manifested by a horse undergoing corresponding immuniza- 

 tion may serve as an illustration of acquired antitoxic immunity. 

 The natural resistance of rats and dogs to anthrax, on the other hand, 

 is of antibacterial character, as is also the immunity or increased 

 resistance, at any rate, which results on vaccination with the 

 same organism in otherwise susceptible animals, such as sheep, 

 guinea-pigs, and mice. 



If an individual becomes immune to a given organism or its toxic 

 products as a result of infection or vaccination, in consequence of 

 his own efforts, as it were, we speak of active immunity, while immu- 



