4 INTRODUCTION. 



antibodies, is injected into another normal but non-immunized animal, the 

 latter acquires the power of being immune against the specific infective 

 agent. In this case the immunity was not established by direct cell activity 

 on the part of the animal, for the organism remained passive, and had, as it 

 were, immunity thrust upon it. This form of immunity in contradistinction 

 to "active immunity" is designated as " passive immunity." 



The forms of immunity thus far mentioned were all "acquired" either 

 by the spontaneous recovery from the infection or the artifical transmission 

 of the curative antibodies. In contrast, however, to this "acquired" immu- 

 nity there is a "natural" immunity by which is understood that some ani- 

 mal species are not at all susceptible to certain infections. Thus man has a 

 natural immunity against a group of diseases markedly fatal for some of the 

 lower animals, e.g., chicken-cholera and hog-cholera. That this natural 

 immunity is almost always cellular in character is undeniably true; and the 

 most important form of this natural armament against infection is the pow- 

 erful leucocyte, capable of engulfing and destroying the invading enemy. In 

 other words, phagocytosis. 



Finally one should speak of a "local" and "general" immunity, mean- 

 ing to express thereby the different resistance and susceptibility that various 

 organs of the same individual display; and also of a "relative" and "abso- 

 lute" immunity in order to differentiate quantitatively a transitory immu- 

 nity from one that is of long duration. 



Another term very often employed is "antibody." This, as 

 Conception k as a } reac [y b een explained, is a name used to designate the 

 Antibod s P ec i nc bodies which the organism produces as a reaction 

 against the infecting agents and their toxic products. Anti- 

 bodies are also formed when animals are injected with foreign proteids not 

 of bacterial origin, such as the blood of a different species of animal, egg 

 albumin, etc. In order that these antibodies may be obtained, the sub- 

 stances employed must enter the system "parenteral," i.e., some way out- 

 side of the gastrointestinal tract. 



In older literature the terms antibody and protective body were used 

 synonymously. That is decidedly incorrect, inasmuch as not all antibodies 

 possess the power of protection and not every actively immune organism, 

 demonstrable antibodies. Furthermore, antibodies as the bacteriolysins 

 which are generally considered to have protective powers, and correctly so 

 too, can exist in a system in large numbers without necessarily rendering 

 that organism immune. 



As an example of how complicated various chapters in the study of immunity can be, 

 will be clearly evidenced by a few of the author's experiments with the hog-cholera 

 bacillus. Rabbits rendered actively immune by inoculation with extracts of hog-cholera 

 bacilli possess a serum which when injected into an animal of a different species as the 

 guinea-pig, will render the latter passively immune. If, however, the serum is injected 



