242 A MANUAL OF BACTERIOLOGY 



effect upon them ; if the toxin were not fixed, it would be 

 possible to detect it, and presumably it would not produce 

 antitoxin. 



2. Natural immunity against micro-organisms. A num- 

 ber of factors are doubtless concerned in preserving the 

 body from invasion by micro-organisms, and while non- 

 specific reactions may suffice when the number of organ- 

 isms is small, specific reactions have to be evoked if the 

 number of organisms be large. The unbroken surfaces' 

 of the body have a considerable protective action in 

 preventing the entrance of micro-organisms. Infection 

 is an active process quite different from the mere presence 

 upon the skin or a mucous membrane of the parasite 

 capable of causing disease. A whole host of potentially 

 infective bacteria are constantly present upon the skin 

 and mucous membranes which for the most part do no 

 harm whatever, and possibly by preoccupation of the 

 soil tend to ward off other more definitely injurious 

 organisms. The surfaces of the body indeed seem to 

 possess a high degree of insusceptibility to ordinary 

 infections ; they have a local immunity. Thus wounds of 

 the mouth and rectum generally heal well in spite of their 

 septic condition. In some cases this local immunity 

 depends upon factors which operate only so long as they 

 are intact. In the young the mucous membrane of the 

 digestive tract is easily affected so as to become the seat 

 of slight pathological conditions that depress its pro- 

 tective power, and hence the prevalence of tuberculous 

 cervical and mesenteric glands and of microbic infections 

 of the stomach and intestine in the young. Resistance 

 to typhoid and paratyphoid infections resides essentially 

 in the mucous membrane of the intestine (Besredka), and 

 typhoid fever ceases with the anatomical changes, coming 

 on in the fourth decade of life, that reduce the absorbing 

 power of the intestine. 



