276 THE HUMAN BODY 



of virulence that " exposure " to an infected individual is so often 

 followed by infection. The fact of increased virulence explains 

 also the occurrence of "epidemics." 



Recovery from Infection involves two processes: 1. Destroying 

 and getting rid of the enormous numbers of bacteria which de- 

 velop during the progress of the disease; 2. Getting rid of or 

 neutralizing the poison, or toxin, which the bacteria produce and 

 which is the real cause of trouble. The course of every infection 

 is a struggle between the Body on one hand and the micro- 

 organisms on the other. The outcome is recovery or death ac- 

 cording as one side or the other proves victorious. 



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. 



The second sort of bodies produced by the living cells under 

 the stimulus of foreign organisms are the immune bodies. These 

 function, as do the intermediary bodies described above, to enable 

 the complements to attack and destroy the invading bacteria, 

 but many times more powerfully. 



In addition to opsonins and immune bodies, the cells under 



