IMMUNITY 73 



is suffering from typhoid fever or has recently had the 

 disease, for the serum continues to show the reaction for 

 some time after convalescence. 



Agglutination reactions may also be employed in a 

 reverse manner for identifying bacteria. In that case, 

 one employs an agglutinating serum made against a 

 certain bacterium and tests the bacterium one is study- 

 ing. If it agglutinates with this serum, one argues that 

 it is identical with, or at least very closely related to, 

 the bacterium used for making the serum. 



Fig. 25.— Typhoid bacilli, showing typic clumping by typhoid 

 serum (Jordan). 



Opsonins. — We have already said that the white 

 blood-corpuscles— 1. e., the leukocytes — take up bac- 

 teria and destroy them. Sir Almroth Wright, a dis- 

 tinguished English physician, discovered that certain 

 substances present in blood-serum had the power of in- 

 creasing the appetite, as it were, of the leukocytes, and, 

 furthermore, that the amount of these sul)stances 

 could be increased by properly administered injections 

 of the appropriate bacteria. These substances he called 



