PRACTICAL APPLICATIONS 365 



According to Casagrandi, non-pathogenic varieties of the 

 pneumococcus form a haemolysin, whilst virulent ones do not. 

 The same observer states that certain cultures produce a leuco- 

 cidin. 



The nature of the immunity to the pneumococcus, and in 

 especial the causation of the crisis (when present), has been much 

 discussed, and now seems to be fairly well elucidated. It is 

 doubtful whether any bactericidal substances are formed in man 

 after an attack of pneumonia ; they may be produced in rabbits as 

 a result of artificial immunization, but are not invariably present, 

 and it is useless to attempt to explain the characteristic crisis by 

 their agency. Anti-endotoxin may perhaps be formed artificially, 

 but its presence in human disease is very doubtful. It is, how- 

 ever, now tolerably clear that recovery in pneumococcic infections 

 is brought about mainly by the action of opsonins, and more 

 especially thermolabile opsonins. This was well shown by 

 MacDonald, whose investigations have been referred to already. 

 The opsonic index is low during the course of the attack, and rises 

 suddenly to normal at or near the crisis. Eyre distinguishes three 

 types of opsonic index in different forms of pneumococcic infec- 

 tions. The first is the pneumonic type, his results precisely 

 corroborating MacDonald ; the second is the form in which 

 recovery takes place by lysis, in which the rise is gradual ; and 

 the third, in which the disease progresses to a fatal termination, is 

 characterized by a steady fall in the index. Eyre holds that the 

 resistance of the individual may be measured by his opsonic 

 response, as represented by these three curves. 



The opsonin present in natural disease is, as mentioned above, 

 mostly thermostable ; but in the immunized sera of hypervaccinated 

 animals thermostable opsonin, the bacteriotropin of Neufeld and 

 Rimpau, is present. In all probability it is to these substances 

 that antipneumococcic serum owes its effects. These are not 

 great according to Eyre, i c.c. of the most powerful serum yet 

 obtained will only protect against some 300 lethal doses as we 

 should expect to be the case if the immunity conveyed were due 

 to a rise in the opsonic value of the blood. 



Agglutinins are usually found in cases of pneumonia, but they 

 are not present in large amount (the serum rarely clumps at a 

 greater dilution than i : 60), though a much more powerful action 

 may be obtained by artificial immune serum. The reaction is of 

 little value in diagnosis, since it may be absent even in pneumonia, 



