Immunity of Rats towards Jensen's Rat Sarcoma. 7 



Eats may be made immune in a similar manner if the sarcoma cells are 

 previously exposed to radium emanation in a concentration of about 0"5 milli- 

 curie per c.c, the exposure lasting about one hour {loc. cit.). The amount of 

 irradiated material inoculated has usually been 0"5 c.c. in the right and left 

 axillae ; after two or three weeks the immunity of the animals is tested by 

 inoculation. 



(c) Condition of Tissues. — A microscopic examination of the spleen shows, 

 in a very large number of striking difference between the normal 



and the immune animal. This contrast is provided by a very large increase 

 in the total number of lymphocytes and plasma cells in the spleen of 

 immune animals compared with that usually obtaining in the normals. 

 (This increase was found to be accentuated in the immune rats after they 

 had received a large inoculation, i.e., 0"3 or 0*4 c.c.) Other observers 

 have shown a similar increase in these cells throughout the connective 

 tissues (2). 



Figs. 4, 5, 6 (Plates 1 and 2) illustrate the contrast referred to in the three 

 types of spleen. 



This contrast in the appearance of the spleen seemed to warrant a more 

 extended investigation. The attempt was made to see whether this massing 

 of the lymphocytes and plasma cells was directly associated with the 

 development of the immune condition. 



In the first place, an examination of the spleen was made in a large 

 number of animals which were in different conditions as regards their 

 toleration to the growth of sarcoma cells. They were classified as Normal (N), 

 i.e., rats practically certain to bear tumours if inoculated ; Immune (I), i.e., 

 rats proved to be resistant to inoculation ; " Massive " (M), i.e., immune rats 

 which had been inoculated with 6 x 0'05 c.c. sarcoma emulsion with negative 

 results ; Progressive (P), i.e., animals which were supporting the growth of a 

 progressively growing tximour ; and Disappearing (D), i.e., rats in which 

 tumours were spontaneously disappearing. 



By giving numerical values, from to 12, to the lymphocyte and 

 plasma-cell content of the spleen, it was possible to classify these various 

 types, and to show that an increased content was not necessarily associated 

 with the immune condition, nor did an immune animal invariably exhibit a 

 high content of these particular cells in its spleen, although this was the 

 usual condition. 



To eliminate all personal bias in the matter, the whole collection of slides 

 (numbering about 300) of the sections of the various spleens was examined 

 without any knowledge on the part of the observer of the particular class to 

 which any slide belonged. Numerical values of the cell content were 



