298 AGEICULTUEAL AND INDUSTRIAL BACTERIOLOGY 



capable of engulfing and destroying foreign cells such as 

 bacteria which may gain access to the body. He conceived 

 immunity, therefore, as being the process of training the 

 white blood cells to engulf and destroy invading micro- 

 organisms. Later two English investigators, Wright and 

 Douglas, showed that the white blood cells by themselves 

 usually do not possess the ability to engulf and destroy 

 microorganisms, but they require the presence of a serum 

 constituent of antibody to which the name opsonin has been 

 given. They showed that immunity in many diseases is due 

 to the development of these opsonins which so change the 

 bacteria that they become attractive or positively ehemo- 

 taetic for the white blood corpuscles. 



Detection of Opsonin. — The fact that white blood cor- 

 puscles have the power of destroying bacteria may be shown 

 by making injections of suitable bacteria into the blood 

 stream of an animal, removing samples of blood at intervals 

 and preparing stained mounts. By the use of an appro- 

 priate dye, the cytoplasm of the white blood corpuscles will 

 stain very lightly, the nucleus of the cell deeply and any 

 bacteria either on the inside or outside of the cell will also 

 stain deeply. It is possible by microscopic observation in 

 this fashion to count the number of bacteria which have 

 been engulfed by a white blood cell. Study will show 

 that the bacteria in the blood will at first be free, later and 

 in larger proportions, they will be found inside of the 

 white blood cells. Finally they will begin to break to pieces, 

 become granular and finally completely disappear. 



Similar observations may be made outside of the body. 

 For this purpose it is necessary to secure suspensions of 

 white blood corpuscles, bacteria and the serum to be tested. 

 The white blood cells are secured by centrifuging blood 

 which has been prevented from coagulating by the addition 

 of sodium citrate. The red blood cells, having a higher 

 specific gravity, settle to the bottom. -The serum remains 



