282 AGEICULTUEAL AND INDUSTEIAL BACTEBIOLOGY 



solve the particular organism in question. A person who 

 has completely recovered from typhoid fever, for example, 

 will generally contain in his blood serum substances which 

 will dissolve or digest typhoid bacilli. Such substances .or 

 antibodies are termed b act erioly sins. Sometimes another 

 type of antibody may be developed against bacterial or 

 other cells. These substances function by making the bac- 

 teria more attractive apparently to the white blood cells. 

 In other words, they enable the white blood cells to destroy 

 the microorganism. They are termed opsonins. Our con- 

 sideration of the factors underlying immunity, therefore, 

 must include a brief consideration of these various anti- 

 bodies and their corresponding antigens. 



Toxins and Antitoxins 



A toxin may be defined as an organic poison, that is, a 

 poison produced by living cells of plants or animals and 

 having certain characteristics of which the following are 

 most important: 



1. Toxins are very easily destroyed (labile). Particularly 

 are they thermolabile, that is, easily destroyed by heat. 

 They deteriorate rapidly when kept in the light and in the 

 presence of moisture. 



2. When toxins are injected into suitable animals in 

 amounts smaller than sufficient to cause death (sublethal 

 doses) these animals will respond by the development in the 

 blood serum of antitoxins or substances which will neutral- 

 ize the toxins. No poison, therefore, is a true toxin which 

 will not cause animals to produce antitoxin to neu- 

 tralize it. 



It will be noted that the above definition does not include 

 all organic poisons. The immunity resulting from the 

 development of antitoxins in the body must be carefully 

 differentiated from so-called drug habituation. Repeated 

 injection of morphine (an organic poison), for example, 



