IMMUNITY 69 



dissolving sera was somewhat complex and required the 

 joint action of two different constituents. One of these 

 constituents decomposes very easily, so that a serum 

 which has stood several days may be found to have 

 almost entirely lost its solvent power. Curiously, how- 

 ever, the addition of a little fresh serum, even from a 

 normal animal, immediately restores its power. This 

 very unstable constituent, which is present in all serum, 

 even in serum from normal animals, is spoken of as 

 complement, because it completes or complements the 

 action of the other constituent. The stable constituent 

 is called the amboceptor or the immune body. 



When the animal is repeatedly injected with gradually 

 increasing doses of bacteria (or other cells), it responds by 

 manufacturing large quantities of this amboceptor or 

 immune body, directed specifically against the injected 

 bacte'ria. This substance lays hold of the invading bac- 

 teria, and, with the aid of the complement, effects their 

 destruction. The complement alone would be unable to 

 destroy the bacteria; the amboceptor is needed to pre- 

 pare the bacteria in some way as yet unknown. 



Agglutinins. When the serum of an animal which 

 has been repeatedly injected with gradually increasing 

 doses of bacteria is brought into contact with some of 

 these bacteria, careful observation under a microscope 

 reveals a very interesting series of changes. Thus, if 

 typhoid bacilli are mixed with a specific antityphoid 

 serum (obtained, say, from a rabbit previously injected 

 with typhoid bacilli), one notices first that the motility of 

 the bacilli becomes markedly diminished. This is fol- 

 lowed by the gradual collection of the bacilli into clumps. 

 At the end of an hour or two, in place of countless bac- 



