IMMUNITY 67 



vading bacteria. These cells take hold of, or rather 

 engulf, the bacteria and digest them.^ 



(6) Still another mode of defence is seen in what 

 takes place in abscesses. When these are examined, it is 

 found that the body has built a wall around the infected 

 area, thus shutting off the bacteria and their poisonous 

 products from the rest of the body. 



Returning now to the mechanism of the specific 

 acquired immunity discussed above, we find that, in 

 response to the invasion by pathogenic bacteria, the 

 body manufactures certain specific substances designed 

 to destroy the invaders or to neutralize their poisonous 

 products. 



These antagonistic substances are spoken of as anti- 

 bodies. The important antibodies thus far known are 

 as follows: 



(1) Antitoxins. 



(2) Bacteriolysins, hemolysins (cytolysins). 



(3) Agglutinins. 



(4) Opsonins. 



(5) Precipitins. 



(6) Antiferments. 



Antitoxins. — When an animal is injected with 

 gradually increasing doses of toxin — e. g., with diph- 

 theria toxin — it will be found that after a time the animal 

 withstands doses of the poison which would suffice to 

 kill hundreds of animals not so treated. This was done 

 in 1890 by von Behring, who found that the blood of the 

 treated animals contained something which neutralized 



^ Certain substances, such as alcohol, also exposure to cold, make 

 the leukocytes sluggish, so that drunkeness on the part of an in- 

 dividual or exposure to cold often lead to bacterial invasion. 



