346 LOCAL LESIONS 



generalized infections. We notice, for example, that in pneumonia 

 it remains at a low level during the attack, and increases rather 

 suddenly at the time of the crisis ; the pneumococci disappear 

 from the blood at this period, though they remain living in the 

 lung until much later. In cases which recover by lysis, on the 

 other hand, there is a gradual increase in the opsonic index, and in 

 acutely fatal pneumococcal septicaemia the index falls gradually 

 and continuously. When dealing with generalized infections the 

 opsonic index (as far as our researches have gone at present) may 

 be taken as a very rough guide to the degree of immunity and 

 chance of recovery. This does not apply in local infections, but 

 here we are probably justified in believing that the higher the 

 index, the less the chance of generalization. It is worthy of 

 notice that the disease cerebro-spinal meningitis in which the 

 highest indices (sometimes as high as ten) are found is one in 

 which bacteriaemia very rarely occurs, though by analogy with 

 other similar diseases we should rather expect it to do so. As 

 regards the source of this increased amount of alexin-opsonin, 

 there is nothing to be added to what has been already stated. 

 (3) In some cases recovery may be brought about by the produc- 

 tion of thermostable opsonins, whether amboceptor (as appears 

 most probable) or agglutinin. This is rarely, if ever, the case in 

 the pneumococcic diseases, but probably occurs constantly in the 

 maladies, like typhoid fever and cholera, in which agglutinins and 

 bacteriolysins are formed in large amount. We may, indeed, 

 classify diseases roughly into two main groups in this respect : 

 (i) those in which the acquired immunity is due (inter alia) to the 

 presence of thermolabile opsonins : in these we may expect the 

 degree of the immunity to be but slight and its duration short; 

 and (2) those in which it is due to true antibodies : here we may 

 expect the opposite conditions to hold. Pneumonia and typhoid 

 fever may be taken as examples. We do not know accurately 

 the duration of the immunity to the latter disease, but the anti- 

 bodies on which we believe it to depend may be traced for long 

 periods, whereas the blood returns to its normal state very shortly 

 after recovery from a pneumococcic infection. And experience 

 with typhoid vaccine leads us to suppose that the immunity to that 

 disease lasts a year or more. The occurrence of relapses may 

 seem to argue that the protection is in reality very evanescent, 

 but other interpretations are possible. 



The processes which occur in local lesions are similar in 



