IMMUNITY TO TYPHOID. 441 



culation. That resistance to typhoid infection is 

 decreased by low nutrition and overwork is a long- 

 known fact. 



A large amount of protection is afforded by the 

 hydrochloric acid of the gastric juice, and it is immunity 

 reasonable to believe that suppression or an insuf- 

 ficient amount of hydrochloric acid may favor the 

 passage of living bacilli to the intestines. Normal 

 human serum is rather strongly bactericidal for 

 the typhoid bacillus, and the leucocytes ingest and 

 destroy it. Metchnikoff ascribes natural immun- 

 ity to the action of the microphages. 



The immunity which follows an attack of Duration of 

 typhoid fever is generally of long duration, but immunity. 

 second attacks occur with some frequency. Accord- 

 ing to Dreschfeld's figures 0.7 per cent, of individ- 

 uals are affected twice. It has been noted that 

 limited communities which have experienced an 

 epidemic may remain relatively free from the dis- 

 ease over a period of some years, although neigh- 

 boring districts are attacked. All the susceptible 

 persons having had the disease, a state of temporary 

 immunity is created. 



Acquired immunity is characterized by an in- 

 crease of the bactericidal amboceptors, opsonins, 

 agglutinins and typhoid precipitins in the serum. 

 It has been shown that recovery is accompanied by 

 an increase in concentration of antibodies. Bac- 

 tericidal amboceptors reach a concentration two or 

 three times that of normal serum and then return 

 gradually to normal, reaching a normal concentra- 

 tion in a few months. Opsonins increase in con- 

 centration as do bactericidal amboceptors, but 

 remain high for many months. Stone believes that 

 this increased power of phagocytosis constitutes 



