UNICELLULAR FORMS 83 



afTect the nervous system more than other tissues, particularly the 

 nerves which cause muscle contraction. Those muscles which close 

 the jaw are noticeably affected; hence the name, lock jaw. Simi- 

 larly, in diphtheria the growth of bacteria is local, usually in the 

 throat, but the toxins are distributed and affect other regions in a 

 violent fashion. 



But it must be remembered that in general bacteremias, the bac- 

 teria, although not localized, are producing toxins; hence bac- 

 teremias or septicemias are in reality special forms of toxemias. It 

 may be remarked here that this action of the parasitic organisms in 

 producing a toxin that destroys its host is a striking example of 

 uneconomic arrangement in Nature, for the parasite is bringing 

 certain eventual destruction on itself bv the destruction of its own 

 food supply. 



Response of Animals to Bacterial Invasions. The host is 

 not entirely helpless against the attacks of bacterial parasites, for the 

 body cells of the host may, and in many cases do, produce sub- 

 stances that neutralize or destroy the toxins produced by the invad- 

 ing bacteria, or destroy the bacteria, or may by the production of a 

 wall about the infection isolate it and thus prevent its spread. Also, 

 a certain type of white blood cell acts as a defense agent and engulfs 

 bacteria and other foreign particles in a manner similar to the en- 

 gulfing of food by amceba. This is an important factor in the defense 

 of the animal; the process is called phagocytosis and this type of 

 blood cell is called a phagocyte. 



Two general types of substances are produced by the host which 

 tend to neutralize the invasion of the bacteria; these are antitoxins, 

 which neutralize the toxins, and bacteriolysins, which destroy the 

 bacteria. Collectively they are spoken of as antibodies. 



Immunity. A general protection against infection by a patho- 

 genic micro-organism is called immunity. Immunity may be of 

 several sorts. 



Natural Immunity: Man is immune to certain diseases of other 



