248 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 above 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 ever}^ evidence that they 

 have had a disease in ntero and are thereafter as immune 

 as though the attack had occurred after birth ("small-pox 

 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). This is of no practical value as yet. 

 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 exceptions. 



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 dis- 

 cussed 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 immunit}^ (preparation of diphtheria and 

 tetanus antitoxin from the horse, immunization of children 

 against diphtheria by toxin-antitoxin injections) . The method 



