26 BACTERIOLOGY 



Agglutinins. There is found in the blood-serum of 

 animals suffering from certain diseases substances which, 

 when such serum is added to a liquid culture of the germ 

 of that disease, cause a clumping together into a motion- 

 less mass of the bacteria of the culture. Advantage is 

 taken of this in the so-called Widal test for typhoid 

 fever, which will be described later. 



Phagocytosis. Metchnikoff in his studies discovered 

 that certain body cells, chiefly the polynuclear leuko- 

 cytes, possess the power of surrounding and ingesting 

 foreign substances, such as carbon, dust, and other for- 

 eign bodies inhaled or driven into the tissues by injury, 

 also bits of degenerated or dead tissue, and, most im- 

 portant of all, bacteria. These cells he called phago- 

 cytes and the process phagocytosis. 



Metchnikoff believed that recovery from disease and 

 subsequent immunity depended on the ability of the 

 phagocytes to destroy the invading bacteria, and that 

 when the phagocytes were unequal to the task the bac- 

 teria conquered and death ensued. It was found, how- 

 ever, that in some instances the leukocytes failed to 

 attack the invading germ. Bail found this to be due par- 

 tially to certain products of bacterial activity which he 

 termed 



Aggressins, the property of which seemed to be to 

 render the bacteria more aggressive, while paralyzing 

 or checking the activity of the phagocytes. 



Opsonins. Wright and Douglas in 1903 found that 



