22 PARASITES IN GENERAL 



developed against the effects of intestinal worms as well as against 

 blood and tissue parasites. 



Artificial Immunity. — In every case the reaction of the body 

 against parasites invading it is due to the presence of some 

 particular substance in the parasite which stimulates the body 

 to react against it. This substance, whatever it may be, is 

 called an antigen. The possibility of acquiring immunity with- 

 out being subjected to the disease hes in the fact that the antigen 

 is also present in parasites which have been weakened, by one of 

 several methods, to such an extent as to be powerless to cause 

 the usual symptoms. It may also be present in the dead para- 

 sites or even in the strained excretions from parasites, as obtained 

 from pure cultures. Vaccinations, in the broad sense, are inoc- 

 ulations into the system of weakened or dead parasites or of 

 their products. The body reacts against the harmless antigen 

 thus injected and antibodies are built up just as if the disease had 

 been actually passed through. Antibodies persist throughout 

 life in the case of some parasites, for several years in others, and 

 for only a short time in still others. When the efficacy of the 

 naturally or artificially acquired immunity is gone, as determined 

 by experimentation, a new vaccination must be submitted to in 

 order to obtain protection. Thus yellow fever immunity, which, 

 however, cannot be artificially produced, normally persists 

 through life; smallpox immunity, as acquired by vaccination, 

 for a number of years; and artificially acquired typhoid immunity 

 for about three years. 



Still another method of inducing immunity is possible. By 

 rendering some susceptible animal very highly immune to a 

 particular parasite by repeated inoculations of virulent germs, 

 its serum becomes so charged with antibodies and so powerful 

 in its action against the particular parasite involved, that a very 

 small quantity of such serum injected into another animal or 

 man is sufficient to give a "passive" immunity — passive be- 

 cause the second animal has taken no active part in the for- 

 mation of antibodies. Such immune serum has been found of 

 value in the prevention and cure of certain spirochete dis- 

 eases as well as a number of bacterial diseases. It has the 

 advantage of causing no discomfort during the development of 

 the immunity, but usually is of shorter duration than " active " 

 immunity. 



