222 GENERAL BIOLOGY OF MICRO-ORGANISMS 



Doubtless, the number and activity of the white blood cells and 

 the nature and amount of bactericidal substances in the blood 

 play a part in some instances. 



Acquired Immunity. Acquired immunity results from some 

 experience affecting the individual, either an infection which the 

 individual has survived or some artificial procedure of immuniza- 

 tion. There are recognized two different kinds of acquired 

 immunity, first, active immunity which is due to the activity 

 of the cells of the individual immunized, and second, passive 

 immunity which is produced by introducing into the body material 

 (blood serum) from another animal, which contains substances 

 conferring at once an immunity upon the new individual. 



Active Immunity. Active immunity may be acquired by an 

 attack of the disease. This immunity may endure for a lifetime 

 in some instances (yellow fever, small-pox, scarlet fever) or for 

 many years (typhoid fever) or it may be very evanescent 

 (erysipelas, pneumonia, influenza). Some diseases were at one 

 time so universal that few escaped them, and individuals used 

 to be purposely exposed or inoculated in order to contract the 

 disease and gain the resulting immunity. Inoculation of small- 

 pox seems to have been practised in China about 1000 A. D. and 

 in India as early as the twelfth century, and it was introduced into 

 Europe in 1721 by Lady Montague and was employed very 

 extensively in Europe and America during that century. 



Active immunity may also be produced without causing a 

 definite attack of the disease. This may be accomplished in a 

 variety of ways. Fully virulent micro-organisms may be intro- 

 duced into a part of the body unfavorable to their development. 

 The subcutaneous injection of cholera cultures according to the 

 method of Ferran and Haffkine has proven to be practically 

 without danger, and results in immunity. The same principle 

 is ultilized in immunizing cattle against pleuro-pneumonia. 1 

 Introduction of virulent organisms in very minute doses has been 

 employed to immunize against rabies (Hogyes method), and against 



1 Kolle und Wassermann, II Auflage, Bd. I, S. 928 (1912). 



