86 ANTIGENS AND ANTIBODIES 



demonstrated by injecting a normal animal with a minimal fatal 

 dose of the corresponding bacteria together with an appropriate 

 quantity of serum obtained from an "immunized" animal. In such 

 an event death does not result, because the animal has here been 

 passively immunized by the serum of the immune animal, and now 

 in turn develops an active immunity as the result of the introduction 

 of the bacteria. 



In a previous chapter we have seen that the bacteriolytic action 

 of normal serum is referable to the associated activity of two sub- 

 stances, viz., the thermolabile complement and the thermostabile 

 amboceptor. On studying a bacteriolytic immune serum in this 

 direction, it may be shown that here also the destructive action upon 

 the bacteria is dependent upon complement and corresponding 

 amboceptor, and that its greater degree of activity as compared 

 with normal serum is altogether owing to an increased content of 

 the latter. 



At a time when the antibacterial action of the normal blood serum 

 was first discovered the question of the origin of the "alexins" was 

 wrapped in complete obscurity. In view of the manner in which the 

 production of the immune amboceptors takes place there can be no 

 doubt that a direct connection exists between their appearance and 

 the introduction of the corresponding bacteria, and upon injecting 

 different animals with different species of bacteria we obtain evidence 

 of a most remarkable specificity in the nature of the response, which 

 one can well compare to the vibratory response which is called forth 

 in tuning forks of different pitch by striking forks of corresponding 

 pitch. 



Further studies in this direction have shown that the appearance 

 of such immune amboceptors takes place according to a fairly definite 

 rule: immediately following the injection a period of latency can thus 

 be observed which lasts for a few days and is then followed by a 

 critical ascent of the curve leading to a maximal point from which 

 there is in turn a corresponding drop which at first is fairly abrupt and 

 later more gradual, and hence a slow return to previously existing 

 conditions. As the same result is obtained after the injection of 

 dead bacteria it is clear that the prolonged effect which follows the 

 introduction of the organisms cannot be referable to possible varia- 

 tions in their number which one might otherwise imagine to be 

 operative on different days and at different hours, nor can the 



