J 



32 LOCAL IMMUNITY 



length, but it is advisable to point out that severe traumatism, in 

 the form of violent laceration and contusion of a part, is an 

 extremely powerful predisposing agent, and that it acts in two. 

 ways, or perhaps more. In the first place, there may be some 

 death of tissues, either in small or large amounts, and in these 

 dead tissues the natural resisting powers are of course in abeyance, 

 so that the bacteria will grow unchecked, as they would in dead 

 culture media ; and, secondly, that the blood does not reach this 

 dead material, and the leucocytes only do so with difficulty. The 

 importance of this is well seen in tetanus. The normal tissues 

 have a considerable degree of resistance to this organism, and 

 infection rarely takes place in a clean incised wound, even in 

 cases in which we can be almost certain that the spores of the 

 tetanus bacillus have been introduced. 



Another cause of reduced local immunity is the action of 

 irritants on the tissues. Here we must distinguish two cases. 

 If the irritant be but mild, it may be actually beneficial ; it 

 causes the earlier phenomena of inflammation which we have 

 previously referred to as being protective, and may tend to raise 

 the resistance of the part in consequence. Thus, according to 

 many observers (who do not agree precisely on the interpretation), 

 the injection of a small quantity of almost any bland (but never- 

 theless foreign) substance into the peritoneal cavity may protect 

 an animal against a lethal dose of a bacterial culture introduced 

 subsequently ; normal saline solution, water, broth, serum, etc., 

 all have this action. But if the irritant be more powerful, so that 

 the tissues are killed and the vessels occluded, or the leucocytes 

 killed, the susceptibility of the region is greatly increased. 

 Chemical antiseptics have this action, especially in certain 

 regions, such as the peritoneum. The same thing may be demon- 

 strated experimentally. Tetanus spores washed free of toxin 

 will not produce tetanus in rabbits, but will do so if an irritant 

 such as lactic or carbolic acid is injected simultaneously. 



A few words may be said here on the phenomena of immunity 

 arid susceptibility in relation to the modifications they cause in 

 the infective processes. Where the immunity is great, or, as we 

 say, absolute, the result of an injection of the infective agent is 

 nil ; there is, of course, some degree of inflammation, but this 

 follows the injection of any fluid, even normal saline solution, and 

 the effect of the bacteria themselves is inappreciable. In this 

 case, therefore, the bacteria are immediately destroyed, and the 



