CHAPTER XIII 

 ON IMMUNITY TO BACTERIA 



WE are now in a position to discuss the mechanism by which a 

 bacterial infection is combated. It must be pointed out in the 

 first place, and kept in mind throughout, that different microbes 

 are dealt with in different ways. Some, such as many of the 

 protozoan blood parasites, may be tolerated, since the tissues of 

 their host are immune to the action of their toxins ; others are 

 removed by phagocytosis, after preparation by opsonins, or perhaps 

 by alexins, amboceptors, or other substances, or possibly without 

 previous treatment ; and yet others may be dissolved by the process 

 discussed under the heading of Bacteriolysis. Each of these 

 processes gives rise to a different form of immunity, which we 

 may call (i) immunitas non sterilisans, (2) phagocytic immunity, 

 and (3) bacteriolytic immunity. And the latter two processes 

 (which are the only ones of practical importance) rarely, if ever, 

 exist in a pure state ; almost all invading bacteria are, or may be, 

 dealt with by a combination of the two. 



We must also notice that the defensive processes will be greatly 

 modified by the nature of the region in which the combat between 

 the invaders and the defensive mechanism goes on. There are 

 three main cases : (i) the circulating blood, (2) the tissues, and 

 (3) regions, such as the peritoneum, combining^some of the 

 characters of the other two. We will take them Jh this order. 



In dealing with the processes that take place when bacteria gain 

 access to the circulating blood, our main difficulty at first sight is 

 not to explain the existence of immunity, but the reason why 

 infection ever occurs ; for the defensive mechanism seems more 

 than adequate to combat any dose of bacteria which is likely to 

 gain access to the blood under natural conditions. The processes 

 by which the bacteria are removed are phagocytosis and bacterio- 

 lysis, or the two combined, and in either case the defence would 

 appear to be more than adequate. Thus, in an experiment for the 



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