4 INTRODUCTION 



If the serum of an animal which has been immunized, and containing 

 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 artificial 

 transmission of the curative antibodies. In contrast, however, to this 

 "acquired" immunity there is a "natural" immunity by which is under- 

 stood that some animal species are not at all susceptible to certain infec- 

 tions. 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 powerful leucocyte, capable of engulfing 

 and destroying the invading enemy. In other words, phagocytosis. 



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

 meaning to express thereby the different resistance and susceptibility 

 that various organs of the same individual display; and also of a "relative" 

 and "absolute" immunity in order to differentiate quantitatively a 

 transitory immunity from one that is of long duration. 



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

 has already been explained, is a name used to designate the 

 Antibody s P ecmc 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 bacteria] origin, such as the blood from a different animal species, 

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

 substances employed must enter the system "parenteral," i.e., some way 

 outside of the gastrointestinal tract. 



In older literature the terms antibody and protective body were used 

 synonymously. That is decidedly incorrect, inasmuch as not all anti- 

 bodies 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 with out neces- 

 sarily rendering that organism immune. 



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 



