ON IMMUNITY TO BACTERIA 347 



nature, but modified by their occurrence in the tissues and not in 

 a fluid medium, and they tend to deviate more from test-tube 

 conditions than do the blood infections. 



At the commencement of the infection the conditions are quite 

 comparable to those of a haemic infection. Leucocytes are soon 

 attracted into the area, and, if the bacteria are not too virulent, 

 may remove them entirely, the infection being thus cut short at 

 its very commencement. Probably myriads of slight wounds are 

 thus dealt with. But when the bacteria resist phagocytosis, and 

 form virulent toxins, a new set of factors are brought into 

 existence. These have been glanced at previously, and will now 

 require further discussion. 



The dilatation of the vessels of the infected region and the 

 consequent acceleration of the blood-flow is, as has been pointed 

 out, altogether a conservative influence, having for its object the 

 removal of toxins and their dilution in the general mass of the 

 blood. But in all cases except the most trivial the local reaction 

 extends further, and the more extensive changes tend, partly at 

 least, to favour the spread of the infection by shielding the 

 bacteria from the full action of the blood. In other words, the 

 middle stages in the evolution of an inflammatory lesion indicates 

 a victory for the bacterium in so far that it has succeeded in 

 altering the tissues to such an extent as to shield itself from the 

 action of the protective forces. At a still later stage (in lesions 

 going on to recovery) the tissues and juices gain the victory, and 

 there is an alteration in the nature of an increase in the local, and 

 usually of the general, immunity. 



Let us trace what we know of this process in the case of a 

 small staphylococcic lesion. The early stages consist of the usual 

 inflammatory reaction. The increased access of blood, increased 

 number of leucocytes, and probably increased amount of opsonin, 

 present in the tissues may suffice to cut short the process, the 

 whole of the bacteria being removed by phagocytosis. In this 

 case the only clinical signs will be those of acute inflammation, 

 resulting in rapid recovery. . 



If, however, the bacteria are present in too great numbers, or 

 are too violent, or if the immunity (local or general) is deficient, 

 the toxins formed will kill the tissues in the neighbourhood, and 

 the bacteria will then lie in a smaller or larger mass of necrotic 

 tissue, which is surrounded by an inflammatory zone. The tide 

 has now turned very definitely in favour of the bacteria, for four 



