ON IMMUNITY TO BACTERIA 349 



become generalized with a high opsonic index, yet even this may 

 occur. We may trace a certain degree of parallelism between the 

 ease of access of opsonin to all parts of the local lesion and the 

 likelihood of a cure following an elevation of the opsonic index. 

 For example, there is, according to Tunnicliffe, a definite relation 

 between the rise of the opsonic index and the disappearance of the 

 membrane in diphtheria, the latter clearing rapidly when the index 

 rises above normal. Here the conditions are these : there is a 

 membrane, a few millimetres thick, composed of dead material, in 

 which the specific bacilli are elaborating their toxins ; below this 

 there is a zone of acute inflammation, hyperaemic and infiltrated with 

 phagocytes. Now it is easier for the toxins to diffuse outward and 

 be washed away by the secretions of the mouth than to pass 

 inwards, and probably but a small fraction actually formed reaches 

 the blood-stream. On the other hand, it is comparatively easy for 

 the protective substances in the serum to pass outwards, the con- 

 ditions being quite different from those in a closed abscess, where 

 opsonins, etc., can only reach the centre of the lesion by diffusion, 

 and not by actual transudation. Here there is a constant stream of 

 lymph from the pervious vessels to the free surface. Thus the 

 low concentration of the toxins renders it easy for the leucocytes 

 to gain access to the bacilli, and the latter have abundant oppor- 

 tunities of becoming opsonized ; hence the conditions for successful 

 phagocytosis are produced. 



A similar train of phenomena follows the free drainage of an 

 acute abscess. The toxins escape outwards, and are no longer 

 forced into the tissues, and at the same time there is a flow of 

 protective lymph into the abscess cavity, and consequent sensitiza- 

 tion of the bacteria, and the removal of the toxins allows the 

 phagocytes to act. If, however, the wall of the abscess is very 

 thick and impermeable, it may be, as in Bulloch's experiment, 

 that the lymph which exudes is deprived of its opsonin and other 

 protective substances during filtration, and the conditions are then 

 less favourable. 



Now consider a large staphylococcic lesion completely embedded 

 in the tissues. There is a large core of dead material, in which the 

 cocci constantly produce toxins, and in which they are completely 

 shielded both from fresh lymph or plasma and from leucocytes. 

 Here we should expect the lesion to be progressive, no matter how 

 high the opsonic index, and this is usually the case. Hence, other 

 things being equal, it is in the smaller lesions that we may expect 



