MAN'S FIGHT FOR SURVIVAL 



627 



person whose blood is tested has typhoid, the bacteria will imme- 

 diately become clumped together or agglutinated, thus showing that 



'i(//M/^/lT////////////////////^, 



■^^, 





■^/////////,y.//^,^M,//y/////yl4^ 



Agglutination test for typhoid. The diagram at the left shows free-swimming 

 bacteria, at the right the bacteria have become clumped together by theagglutins 

 produced by the body cells. 



the antibodies are already formed and are at work. Just as each 

 disease is caused by a specific kind of organism producing a specific 

 type of toxin, so the blood forms a specific type of antibody for each 

 toxin. 



Another method of receiving immunity has been recently discovered 

 independently by two investigators, Twort and d'Herelle. The 

 latter made a suspension of feces from a convalescent case of dysen- 

 tery, filtered the material, and then added the filtrate to a broth culture 

 of dysentery and found that some substance in the filtrate killed 

 the bacteria. This substance he called bacteriophage. It is ultra- 

 microscopic, specific, being produced by specific bacteria, and appears, 

 under certain conditions, to produce immunity to specific diseases. 



Active Acquired Immunity 



It has long been known that immunity can be acquired through an 

 attack of a given contagious disease. The idea underlying this type 

 of immunity, later developed by Pasteur, is that the causal organism 

 may be weakened, then inoculated into a person's body, and a 

 slight attack of the disease thus induced. Active immunity is now 

 brought about in different ways through the introduction of (1) living 

 organisms causing the disease, (2) attenuated or weakened organisms, 

 (3) dead organisms, or (4) extracts of products of the organism. All 

 of these substances may be called vaccines. The underlying prin- 



