230 IMMUNITY 



Immunity present in an animal and not due to human 

 interference is to be regarded as natural immunity, while if 

 brought about by man's effort it is considered artificial. 

 Those cases of natural immunity mentioned abo\'e which 

 are common to divisions, classes, orders, families, species or 

 races of organisms and to those few individuals where no 

 special cause is discoverable, must be regarded as instances 

 of true inheritance through the germ cell as other char- 

 acteristics are. All other kinds of immunity are acquired. 

 Occasionally young are born with every evidence that they 

 have had a disease in utero and are thereafter as immune as 

 though the attack had occurred after birth ("smallpox 

 babies," "hog-cholera pigs"). Experiment has shown that 

 immune substances may pass from the blood of the mother 

 to the fetus in utero and the young be immune for a time 

 after birth (tetanus). It is a familiar fact that with most 

 infectious diseases recovery from one attack confers a more 

 or less lasting immunity, though there are marked excep- 

 tions. 



Active Immunity. — By active immunity is meant that 

 which is due to the actual introduction of the organism, or 

 in some cases of its products. The term active is used because 

 the body cells of the animal immunized perform the real 

 work of bringing about the immunity as will be discussed 

 later. In passive immunity the blood serum of an actively 

 immunized animal is introduced into a second animal, 

 which thereupon becomes immune, though its cells are not 

 concerned in the process. The animal is passive, just as a 

 test-tube, in which a reaction takes place, plays no other 

 part than that of a passive container for the reagents. 



In active immunity the organism may be introduced in 

 what is to be considered a natural manner, as when an ani- 

 mal, becomes infected, has a disease, without human inter- 

 ference. Or the organism may be purposely introduced to 

 bring about the immunity. For certain purposes the intro- 

 duction of the products of the organism (toxins) is used to 

 bring about active immunity (preparation of diphtheria and 

 tetanus antitoxin from the horse). The method of produc- 

 ing active immunity by the artificial introduction of the 



