278 AGEICULTUEAL AND INDUSTEIAL BACTERIOLOGY 



sufficient presence of the appropriate type of antibody in 

 the blood or in the body tissues. This theory has apparently 

 stood the test and there is little reason to doubt that im- 

 munity in many diseases, probably in most, is due to the 

 presence or to the development of antibodies which can 

 specifically antagonize the invading microorganism. 



Metschnikoff's Theory of Phagocytosis. — As a result 

 of the microscopic examination of the blood of animals 

 which had been injected with various kinds of bacteria, 

 Metschnikoff came to the conclusion that certain body cells, 

 particularly certain types of white blood corpuscles, were 

 capable of engulfing and destroying invading microorgan- 

 isms. He concluded that immunity was due to the power of 

 these cells to destroy bacteria. An individual who became 

 immune to disease had, so to speak, trained his phagocytic 

 body cells to destroy the particular type of organism in ques- 

 tion. Such an individual, therefore, would be immune to the 

 disease as long as this function on the part of the white 

 blood cells persisted. It will be shown later that Metschni- 

 koff's theory contained much of value. Undoubtedly the 

 white blood cells are important in preventing disease, and 

 any adequate explanation of immunity must take into con- 

 sideration their activity. However, it has been found that 

 the white blood cells by themselves usually are relatively 

 incapable of destroying a microorganism but must act in 

 conjunction with certain antibodies present in the blood 

 stream. It will be noted, therefore, that Ehrlich's theories 

 and Metschnikoff's theories have been finally united and 

 together constitute the basis for the modern science of im- 

 munology. 



Kinds of Immunity. — Common observation teaches that 

 man is immune to many diseases which attack animals. For 

 example, man does not contract hog cholera or dog dis- 

 temper. Most diseases of plants are not transmissible to 

 animals or vice versa. Many diseases of man cannot be 



