480 MICROBIOLOGY 



Natural immunity. Every species of animals possesses 

 resisting powers against certain disease producing microorgan- 

 isms. Occasionally this natural resisting power is present with 

 certain races only. Thus the Algerian sheep are immune to 

 anthrax while other sheep are very susceptible to it. Cattle 

 are immune to glanders and horses do not suffer from black 

 quarter.' 'A satisfactory explanation for this natural immunity 

 has not been recorded. 



Acquired immunity. Acquired immunity is a resistance 

 brought about in the individual by several means, namely : (1) 

 by recovering from a naturally contracted attack of the dis- 

 ease; (2) by inoculating the individual with non-lethal doses 

 of the virulent organisms; (3) by the injection of the indi- 

 vidual with attenuated virus as in vaccination; (4) by inject- 

 ing dead (killed) bacteria or the product resulting from their 

 multiplication in culture media; and (5) by the injection of 

 the blood serum of animals that have recovered from the dis- 

 ease or that have been immunized by any of the above men- 

 tioned methods. This form of immunity varies in its com- 

 pleteness and duration according to the disease and the extent 

 to which the immunizing agents have been applied. 



Explanation of acquired immunity. A number of in- 

 genious explanations have been offered for acquired immunity. 

 The only ones that have withstood the test of their objectors 

 are those of Metchnikoff, representing the cellular theory, and 

 of Ehrlich, representing the humoral theory. The results from 

 different lines of investigation indicate that the factors in- 

 volved in securing immunity against infectious diseases are 

 multiple in number and varied in character. As pointed out 

 by Meltzer it is difficult to explain immunity as being due to 

 one or even to a few anti-bacterial properties of the animal 

 body. In the struggle against bacteria the defence of the body 

 is carried on by the united action of each and every resisting 

 influence. However, a number of theories have been proposed. 



1. The exhaustion theory. This theory was suggested 

 in 1880 by Pasteur, who thought that the microorganisms grow- 

 ing in the body used up some substance essential to their 



