460 BAGTEBIOLOGY. 



gested by Hankin. If the tissue-elements are not of 

 sufficient vigor to neutralize the bacterial poisons, the 

 bacteria are victorious and infection results; while, if 

 there be failure to establish a condition of disease, the 

 tissues are victorious, and are said to be resistant or to 

 possess immunity from this particular form of infection. 



It is a common observation that certain human beings 

 and animals are more susceptible to the different forms 

 of infection than are others, and that some species of 

 animals are apparently not at all liable to particular 

 diseases; in other words, they are naturally immune from 

 the maladies. The term ''natural immunity," as now 

 employed, implies a coQgenital condition of the individ- 

 ual or species, a condition peculiar to his idioplasm, 

 which has been transmitted to him as a tissue-char- 

 acteristic through generations of progenitors. 



Again, it is often observed that au individual or ani- 

 mal after having recovered from certain forms of infec- 

 tion has thereby acquired protection from subsequent 

 attacks of like character; in other words, they are said to 

 have acquired immunity from this trouble. "Acquired 

 immunity" implies, therefore, a condition of the tissues 

 of an individual, not of necessity peculiar to other mem- 

 bers of the race or species, that has resulted during his 

 life from the stimulation of his integral cells by one or 

 another of the specific, infective irritants that may have 

 been purposely introduced, or accidentally gained access 

 to his body. 



The problem involving the explanation of these inter- 

 esting observations has afforded material for reflection 

 and hypothesis for a long time. It is only through 

 investigations that have been conducted during the past 

 few years that it has met with anything approaching 



