IMMTNITY 



amoeboid properties, and are able to invest solid bodies 

 like the bacteria and digest them. This process is known 

 as phagocytosis, and such devouring cells are called phag- 

 ociftcs. Whenever any portion of the body is invaded by 

 a foreign agent, or when any abnormal condition arises, the 

 phagocytes are attracted to this point as a result of a chem- 

 ical stimulus. This causes them to accumulate at or near 

 the point of invasion, where they soon engulf and destroy 

 many of the invading organisms. If these white blood cor- 

 puscles are able to overcome the harmful bacteria, the ini- 

 tial infection may .he rendered of no importance. Again, 

 the organism may multiply and form poisonous products 

 which may injure the body to the extent that death is 

 caused, or, under the stimulus of these harmful products, 

 the body cells may react and form substances that neutralize 

 or nullify in some way the poisonous effects of the products 

 of the organism. These antagonistic and protective pro- 

 ducts are diffused through the liquids of the body, especially 

 in the blood serum and form the. basis of the anti-serums 

 that are used for protective purposes. 



Immunity. A specific disease may occur only in a single 

 host species, as in hog cholera, or it may be capable of 

 spreading throughout a variety of different animals. Black- 

 leg affects only cattle and sheep, while the anthrax bacil- 

 lus produces a characteristic disease in man as well as in 

 many of the lower animals. Numerous diseases affecting 

 man. such as typhoid fever, diphtheria, yellow fever, and 

 cholera, an- limited to this host alone. Other warm-blooded 

 animals are naturally insusceptible to these maladies; they 

 possess a natural immunity. 



The term natural immunity is applied to that condition 

 which enables an animal to resist the natural invasion by 

 organisms that attack other varieties or species of animals. 

 It is a condition that is present at birth, continues through- 



