10 



immunity was produced not only against Type I but also, though 

 in variable degree, against other types of pneumococci. This, 

 they say, " confirms the fact already established that the various 

 " types of pneumococci are closely related biologically." They 

 also state that monkeys, which had survived experimental 

 infection with Type I and were immune to subsequent infectio 

 with this type, sometimes, though not invariably, showed " a 

 '• certain amount of cross-immunity against the other fixed types." 

 But these results did not apply to Type IV. Experimental 

 pneumonia due to this type " confers slight if any protection 

 " against subsequent infection with the same or with an homolo- 

 " gous strain of Pneumococcus Type IV." And Type IV pneu- 

 monia did not confer any immunity against Type I nor, conversely, 

 did Type I pneumonia immunise against Type IV. In their 

 monkeys which had been immunised, either by vaccination or by 

 recovery from experimental pneumonia, there was no constant rela- 

 tionship between active immunity and the presence of agglutinins 

 or protective substances in the animal's serum. " The serum of a 

 " monkey may be entirely free from these substances, and yet the 

 " animal may possess a high grade of immunity against pneumonia. 

 " On the other hand, . . . the serum of a vaccinated monkey 

 " might protect mice against 100 or even 1,000 minimal lethal 

 " doses of pneumococci, and still that monkey be susceptible to 

 " experimental pneumococcus pneumonia. These facts complicate 

 " the whole question of resistance to pneumococcus infection and 

 " revive the old problem of humoral versus cellular immunity." 



To these observations may be added a short note on the later 

 work of Cecil and Steffen.* They found that the subcutaneous 

 inoculation of monkeys with three large doses of Type I vaccine 

 conferred complete immunity against experimental Type I 

 pneumonia, and that the same results were obtained with small 

 doses of the vaccine when used intravenously. But the appear- 

 ance of protective bodies for mice in the serum of their immune 

 monkeys was quite irregular and might not occur. " There 

 " appears," they say, " to be no intimate relation between active 

 " immunity against pneumonia and the presence or absence of 

 " protective substances in the serum of the vaccinated animal." 



The above observations, which suggest that serological re- 

 actions do not furnish a complete explanation of the mechanism 

 of immunity, correspond in some respects, though not in every 

 detail, with the earlier results of Cole and Moore. These investi- 

 gators! found that animals whose sera were protective and 

 curative were always highly immune ; but " an animal may 

 " itself be fairly highly immune without the serum containing any 

 " immune bodies that we can demonstrate, and without its having 

 " any demonstrable protective action." For example, they im- 

 munised rabbits to withstand 0*1 c.c. of a culture, of which 



* Joum. K.rprr. Med., XXXIV, p. 245. 1921 

 f Journ. Exper. Med., XXVI, p. 537. 1917. 



