DISEASE-RESISTING FUNCTIONS OF THE BLOOD 309 



For destroying and getting rid of the bacteria the Body makes 

 use of the same structures, the complements and phagocytes, 

 that it uses in resisting infection in the first place; but the effi- 

 ciency of these is enormously increased through the development 

 of special aids to their activity. 



Opsonins, Immune Bodies, and Agglutinins. The presence and 

 growth of foreign organisms stimulate the cells of the Body to 

 produce and set free in the blood large numbers of bodies of prob- 

 ably at least three sorts. The first of these, called opsonins, act 

 upon the invading bacteria in such a way as to increase very 

 greatly the "appetite" of the phagocytes for them. It is possible 

 to obtain living phagocytes in salt solution, free from the other 

 elements of blood. If to a slide containing some of these a num- 

 ber of bacteria be added and the whole kept at body temperature, 

 the average number of bacteria ingested by each phagocyte can 

 be determined by actual observation. It is found that if the 

 bacteria, before being placed on the slide, are treated with a 

 liquid containing the proper opsonin, the average ingestion per 

 phagocyte is multiplied many fold. It is necessary, for this effect 

 to be produced, that the opsonin be applied to the bacteria; treat- 

 ment of the phagocytes with opsonin, with subsequent washing, 

 does not increase at all their tendency to ingest bacteria. 



Under the stimulus afforded by the presence of foreign or- 

 ganisms are produced, also, great quantities of the special immune 

 bodies needed to give the complements access to those particular 

 organisms. Thus a defensive agency which if present at all before 

 the infection was only slightly effective becomes the chief reliance 

 of the Body in its struggle to rid itself of the invaders. 



In addition to opsonins and immune bodies, the cells under 

 bacterial stimulation produce what is thought to be a third sub- 

 stance, agglutinin, which causes the bacteria to clump together, 

 becoming thus more subject to the action of the phagocytes or 

 complements. The development of these various bodies is the 

 process of immui^zation. 



Antitoxin. Beside the destruction of the invading bacteria 

 it is necessary, before the Body is cured of an infection, that 

 the toxins produced by the rapidly multiplying organisms be neu- 

 tralized. This neutralization of poison is a simpler process than 

 the destruction of formed elements as carried on by the phago- 



