508 IMMUNITY 



develop an active immunity which lasts for life. (2) By vaccina- 

 tion. The introduction of dead or attenuated (weakened) germs 

 of the disease, in small doses, stimulates the body to produce its 

 own antibodies. In vaccinating against typhoid and yellow^ fever, 

 the dead organisms are used. In smallpox, germs weakened by 

 passing them through a cow are used to make up the vaccine. 

 This is known as animal passage. In rabies, the germs are 

 attenuated by drying them. Germs may also be weakened by the 

 application of slow heat or by growing them on media that are 

 not quite favorable. In each of these cases the body cells respond 

 to the inoculation by producing antibodies. (3) By the injection 

 of toxin from which the bacteria have been filtered. Sometimes the 

 toxin has some antitoxin mixed with it to dilute and make it 

 safer, as in diphtheria toxin-antitoxin. Sometimes small quan- 

 tities of the toxin itself are used, as in scarlet fever immunization. 

 Here, again, the body produces antibodies. All actively acciuired 

 innnunity is usually lasting in its effects. Active imnnuiity tjikes 

 some time to })roduce because the cells require time to make 

 their reactions. 



Passive acquired inmiunity is obtained by the injection of anti- 

 toxins or immune serums from the body of another person or an 

 animal. Such innnunity is immediate in its effects, but it does not 

 last for very long. The antitoxin is already prepared. It sets to 

 work promptly neutralizing the toxin present. The injection of 

 such material does not stimulate the body to j)r()duce its ow^i 

 antibodies and hence this imnuniitv lasts for onlv a short time. 

 It is used in the actual treatment of the disease or to })r()tect some 

 one who has been exposed. For example, diphtheria patients and 

 their families are given antitoxin. An immune serum is frequently 

 given for pneumonia and infantile paralysis. 



The immunity of to-morrow. A very recent investigator, d'He- 

 relle, has demonstrated the presence of what he has called a bac- 

 teriophage, a kind of super bacteria that destroy other bacteria 



