6 VACCINE AND SEEUM THEEAPY 



and other germs, the so-called ** hemolysins," attack 

 primarily the red blood corpuscles. Others, again, act 

 upon the white blood cells. We have seen that the 

 mere entrance of pathogenic organisms into the bodies 

 of animals does not always lead to infection. It is 

 plain, therefore, that the animal body has some 

 subtle means of defence, by virtue of which patho- 

 genic germs are, even after their entrance into the 

 tissues and fluids, disposed of, or at least prevented 

 from proliferating and elaborating their poisons. The 

 power which enables the animal to accomplish this is 

 spoken of as resistance. When this resistance, which 

 in a degree is common to all animals, is specially 

 marked, it is spoken of as " immunity." Absolute 

 immunity is exceedingly rare. The entire insuscepti- 

 bility of cold-blooded animals under normal conditions 

 with even the largest doses of many of the bacteria 

 pathogenic to the warm-blooded animals is an 

 instance. 



The power of resisting any specific infection may 

 be the natural heritage of a race or species, and is 

 thus spoken of as "natural immunity." It may, on 

 the other hand, be acquired, either accidentally or 

 artificially, by a member of an ordinarily susceptible 

 species, and is then called, '' acquired immunity." 



Natural Immunity. — It is well known that many 

 of the diseases occurring in the human species do not 

 infect the lower animals. Conversely, there are 

 diseases of the lower animals which cannot be trans- 

 mitted to man. Among the lower animals great 



