478 IMMUNITY 



addition of fresh peritoneal fluid or normal serum to the heated 

 immune-serum. Pfeiffer also found that an anti-serum heated to 

 70 C. for an hour produced the reaction when injected with the 

 corresponding organisms into the peritoneum of a fresh animal. 

 The outcome of these and subsequent researches is to show that 

 when an animal is immunised against a bacterium a substance 

 appears in its serum with combining affinity for that particular 

 organism. This substance which is generally known as the 

 immune-body, amboceptor (Ehrlich), or substance sensibilisatrice 

 (Bordet) is comparatively stable, resisting usually a temperature 

 of 70 C for an hour. It cannot produce the destructive effect 

 alone but requires the addition of a substance normally present 

 in the serum, which is spoken of under various names com- 

 plement (Ehrlich), alexine or cytase (French writers). The com- 

 plement is relatively unstable, being rapidly destroyed by a 

 temperature of 60 C., and it is not increased in amount during 

 the process of immunisation. Though ferment-like in its in- 

 stability, it differs from a ferment in being fixed or used up in 

 definite quantities. 



The phenomenon of lysogenesis is, however, only seen in the 

 case of certain organisms when an animal is highly immunised 

 against them ; the typhoid and cholera group are outstanding 

 examples. It is also to be noted that it sometimes is seen in the 

 case of a normal serum (vide Natural Immunity). In other cases 

 the bactericidal effect of a serum may occur without the rapid dis- 

 solution characteristic of lysogenesis though other structural 

 changes may be produced. In still other cases a bactericidal 

 effect may be wanting ; nevertheless it may be shown that an 

 immune-body is developed by the process of immunisation. This 

 may be done by observing the increased amount of complement 

 which is fixed through the medium of the anti-serum (immune- 

 body), sensitised red corpuscles being used as the test for the 

 presence of free complement. The following scheme will show 

 the mode of experiment, which is carried out in a series of 

 small test-tubes : 



(1) Bacteria + immune-body (anti-serum heatedat 55 C.) + complement. 

 (The same amount of bacteria and immune-body in each tube, varying 

 amounts of complement in different tubes.) 



(2) Incubate at 37 C. for one and a half hours. 



(3) Add to each tube red corpuscles treated with the corresponding 

 immune-body, and incubate for another hour. 



The " immune-body " is in each case the anti-serum deprived 

 of complement (by heating at 55 C.), obtained from animals 

 injected with the bacteria and red corpuscles respectively. The 



