■threshold value," i.e., a certain concentration in proportion 

 t«. the body weight of the animal, before it exercised its protective 

 action. When this quantity of serum was introduced, protection 

 Was valid against a very largo multiple of the minimal fatal dose. 



From their animal experiments, they thought it possible 

 that cases of pneumonia in man might be successfully treated 

 with antipneumococcus sera, provided that three conditions 

 Were fulfilled. (1) The serum must be of high titre in protection 

 tests on mice; (2) it must be prepared with a pneumococcal 

 strain of the same serological type as the strain infecting the 

 patient : and (3) it must be injected intravenously in large doses, 

 e.g., 75 c.c, in order to attain the requisite concentration in 

 the patient's body. They considered that it would be wise to 

 concentrate first on the serological treatment of infections due 

 to their " typical " group of pneumococci, and to wait until the 

 value of this method was established before attempting similar 

 work on infections where the organism was one of the " atypical " 

 varieties. 



They offered the above suggestions as a starting point for 

 future enquiries and made it clear that they were not, at the 

 time of writing (1912), in a position to express any judgment 

 upon the value of serum therapy in the treatment of pneumonia. 



Progress. 

 American Types. 



This starting point would have led to very encouraging 

 results if, on more extensive enquiry into cases of pneumonia, 

 N( utVld and Handel's tentative and cautiously worded suggestions 

 had been substantiated in three respects. It might have been 

 found (1) that " Pneum. I " was responsible for the majority of 

 cases of pneumococcal pneumonia, (2) that, in the remaining 

 cases, the pneumococci present were divisible into a reasonably 

 small number of serological groups, and (3) that, for each of these 

 groups, as for " Pneum. I," specific sera could be produced which 

 were effective not only in protection tests on mice but also in 

 the treatment of human pneumonia. Unfortunately, these 

 anticipations have only been realised to a limited extent. 



Americarj investigators have found that the most important 

 group of strains in their country is Type I, corresponding to 

 Neuf eld's "typical strains." In 1!H7 the Rockefeller Institute 

 reported that this type occurred in 33-3 per cent, of a series 

 of 454 cases of lobar pneumonia. The serological characters of 

 this type are sharply defined; it is definitely associated with 

 disease and is very rarely found in normal individuals, except in 

 contacts with cases of pneumonia. The next American group 

 Type II (typical)." This is serologically distinct from I, hut 



rabies it in it- definite, and almost exclusive, ition 



with disease. It accounted for 29*3 per cent, of the above 



«s of lobar pneumonias. Then there come a large number 



