194 PASSIVE IMMUNIZATION. 



into an animal, the virulence is usually increased thereby only for the respec- 

 tive animal species, but may at the same time be lowered for man. 



Many authors, therefore, employ for the production of immune sera only 

 virulent strains of bacteria freshly isolated from man. 



In spite of all the above considerations, the fact still remains that most 

 immune sera excepting those of the cholera, typhoid, and paratyphoid 

 bacteria, show no bacteriolytic tendencies even under the most favorable 

 circumstances; but by means of their amboceptors they fix free complement 

 and with the aid of bacteriotropins, stimulate phagocytosis. 



Whether complement fixation is at all to be considered as a protective 

 phenomenon, cannot with the presently existing evidence be definitely 

 decided. 



Conditions are much more favorable as far as the bacteriotropins are 

 concerned. Active phagocytosis is always an expression of good resistance 

 power. It is not necessary for the leucocytes to digest the bacteria; it is 

 amply sufficient if a protective wall of these cells is formed (Ribbert, 

 Citron, Gruber); moreover they can neutralize the bacterial poisons. In 

 this connection it must always be borne in mind that phagocytosis by no 

 means necessitates the death of bacteria. 



Granting, however, that all the above requirements have been fulfilled 

 and a suitable serum has actually been produced, will such a serum always 

 be effective, or are there any other causes which may interfere with its good 

 results? In order to answer this, the infectious diseases must be divided 

 into acute and chronic. With the first class, success is quite assured as long 

 as it is possible to bring sufficient amounts of the active serum substances 

 into direct contact with the bacteria. In meningeal infections, intraspinal 

 injections may have to be adopted. It is difficult, however, in cases of 

 this nature to judge definitely whether the serum therapy was really the 

 effective agent, inasmuch as diseases like erysipelas, meningitis, pneu- 

 monia, etc., are self limited, lasting for a period of time and then subsiding 

 of their own accord. 



With the chronic infections, on the other hand (especially tuberculosis), 

 serum therapy has a new difficulty to overcome. As a result of the long 

 duration of the disease, it is naturally impossible by means of a single injec- 

 tion to introduce sufficient curative bodies, as can be accomplished in diph- 

 theria, for example. It is necessary, therefore, to repeat the injections for a 

 long period of time. Under such conditions the human organism produces 

 antibodies against the foreign proteid, perhaps even against the curative 

 substances in the serum (antiamboceptors) . In both instances the desired 

 effect of the serum is lost. 



The interaction between the injected serum and the bodies produced by 

 the organism immunizing itself against it can manifest itself in various clin- 

 ical symptoms known as the " hypersusceptibility " reaction, or the "serum 



