380 GENERAL PRINCIPLES 



view of the host and has led to several very important general 

 conclusions concerning the degree of susceptibility of species or 

 of individuals to given bacterial diseases. From what is said 

 concerning the physiological processes of bacteria in the Ap- 

 pendix to Part I, it may be suspected that the effect produced 

 on the host by the bacterial parasite is due to substances 

 secreted by the bacteria. As a matter of fact, it is found that 

 if an extract from the bacterial cultures containing no living 

 cells is introduced into the body of the host, the symptoms 

 peculiar to the corresponding disease are produced. The bac- 

 terial products are similar to poisons in their effects, and are 

 called toxins. 



775. Immunity. — When an individual or a species is not 

 susceptible to the attack of an infectious disease, it is said to be 

 immune. The immunity may be a native character of the 

 animal, it may be acquired during the life time of the individ- 

 ual through natural causes, or it may be induced artificially. 

 These types are, therefore, designated natural immunity and 

 acquired immunity, respectively, and of the latter there are 

 two types, active and passive. 



776. Natural immunity is due primarily to three kinds of 

 defense, which the organism employs to defend itself against 

 bacteria which have succeeded in entering the body, (i) Any 

 foreign particles introduced into the tissues and causing irri- 

 tation are attacked by the white blood corpuscles. These 

 cells are capable of independent locomotion through amoeboid 

 movements. They escape from the blood vessels by pene- 

 trating the walls and move about in the lymph spaces in the 

 tissues. They collect about foreign matter and engulf and 

 digest particles as would an amoeba. This process of phagocy- 

 tosis is regarded as of great importance in keeping the body 

 free of bacterial invasions. (2) The blood of a naturally 

 immune animal contains a substance, alexin, which causes the 

 death of the bacteria. This substance is probably formed by 



