INTRODUCTORY AND GENERAL 29 



of diseased animals) may be mixed with the serum from hyper- 

 immunized animals and injected subcutaneously. If the serum 

 and virus are injected separately the animal will in all cases 

 acquire passive immunity ; but unless there is some degree of ill- 

 ness (a " reaction") this will be but temporary, and no active 

 immunity will be superadded. Thus, if the serum be injected and 

 the virus given subcutaneously at the same time, no reaction 

 follows, and the immunity does not last more than a month ; but 

 if the injection is made into a vein a reaction occurs, and active 

 immunity, lasting for about a year, will follow (Stockman). 



The method is also used in the early stages of antitoxin forma- 

 tion, the horse being treated with a mixture of toxin and anti- 

 toxin, the latter being in excess. But here it seems unquestionable 

 that active immunity is acquired, and the mechanism by which 

 this occurs is discussed subsequently. 



FIG. 4. MIXED IMMUNITY. 

 The presence of a negative phase, as shown in the diagram, is not essential. 



LOCAL IMMUNITY. We have hitherto spoken of the body as a 

 whole, assuming that all parts are equally resistant or susceptible. 

 This is not the case, and certain parts are found to have a marked 

 degree of immunity to certain bacteria. Here we have to be sure 

 that we are dealing with regions that are equally exposed to 

 infection. The stomach, for example, is comparatively rarely 

 attacked by infective processes, and this may be due to the fact 

 that the gastric juice is of a sufficient degree of acidity to kill or 

 inhibit most bacteria. Yet here it is probable that this does not 

 account for all the phenomena, and that some degree of true local 

 immunity does exist. Numerous other examples may be quoted. 

 Pneumococcic infections are common in the lungs and pleura, but 

 rarely spread further, and cause disease of the ribs and intercostal 

 muscles ; tubercle is common in the bones and extremely rare in 

 the muscles, whilst Trichina spimlis affects the muscles and never 



