ACQUIRED IMMUNITY 251 



hypothesis of immunity). It is probable that the greater 

 part of phagocytosis takes place in the spleen, and dogs 

 deprived of the spleen become susceptible to anthrax. 

 This organ acts as a sort of filter, and phagocytosis 

 may be active in it when none can be discerned in the 

 blood. Phagocytosis is also active in the bone-marrow. 



Although small amounts of antitoxin may occasionally 

 be met with in the normal animal (e.g. diphtheria anti- 

 toxin in man and in the horse ; see pp. 182 and 329), this 

 substance plays little or no part in natural immunity 

 against either toxin or micro-organism. Thus the blood- 

 serum of the fowl, which is highly refractory to tetanus, 

 does not exert the slightest antitoxic or neutralising 

 action on tetanus toxin. 



3. Acquired immunity. Acquired immunity may be 

 induced in several ways : 



(1) By an attack of the disease ending in recovery. 



(2) By introducing the virulent virus in very small 

 amount, or by a special route. 



(3) By vaccinating with a modified and less virulent 

 form of the living infective agent (Pasteur's method). 



(4) By treatment with sterilised cultures, or with 

 bacteria-free toxins. 



(5) Occasionally by treatment with sterilised cultures 

 or toxins of a different species. Thus, B. pyocyaneus 

 protects from anthrax, and Klein asserted that the 

 bacterial protein of any one of the six following organisms 

 (1) Koch's comma, (2) Finkler-Prior's comma, (3) B. 

 coli, (4) Proteus vulgaris, (5) B. prodigiosus, (6) B. 

 typhosus, protects an animal against any of the remaining 

 five, and concluded that there is an immunising agent 

 common to the cells of all these six organisms. In this 

 case, however, the immunity is probably one against 

 certain bacterial proteins and not against the specific 

 endotoxins of the organisms. 



