INTRODUCTORY AND GENERAL 3! 



crops of boils it will often be found that one is undergoing involu- 

 tion whilst another is developing ; hence the cure of the first 

 cannot be due to any general immunity, but must depend on local 

 changes which do not affect the second. A similar line of argu- 

 ment will show the development of acquired immunity to the 

 streptococcus in erysipelas ; the healthy skin is susceptible, since 

 the disease spreads to it, but the process does not extend back- 

 ward into an area already affected, but now cured, or does so but 

 rarely. 



The subject cannot be discussed further with advantage, and 

 will be deferred to a subsequent chapter, when the known factors 

 on which immunity depends have been elucidated. 



There are important non-specific causes for alterations in local 

 immunity, as is the case with general. These practically resolve 

 themselves into the presence or absence of an adequate supply of 

 blood ; the more copious the supply of healthy circulating blood, 

 the greater the resistance to infections, and vice versa. Hence the 

 utility of fomentations and other hot applications in the initial 

 stages of an infective lesion ; hence, too, the application of Bier's 

 method of passive congestion, in which an excess of blood 

 (though partly stagnant) is made to flush the tissues. And there 

 is no doubt that the object of the dilatation of the vessels and 

 acceleration of the flow of blood through them which occurs in 

 the early stages of inflammation is a beneficial process which 

 has this improvement of the local resisting powers as one of its 

 objects, the influx of an increased number of leucocytes and the 

 dilution and removal of the soluble toxins being others. In acute 

 inflammation we may distinguish two stages. In the first, the 

 stage just mentioned, the conservative reaction of the vessels is 

 most obvious, and in the case of a mild infection, or if the 

 immunity is very strong, may suffice to destroy and remove the 

 infective material and its toxin. The stagnation and ultimate 

 cessation of the blood-flow are indications that the irritant is, 

 temporarily at least, getting the upper hand, and, by cutting off 

 the blood-supply, is neutralizing the most powerful defensive 

 factor. The acceleration of the flow may be regarded as physio- 

 logical, the retardation and cessation as pathological. 



The causes of local reduction of immunity by obstruction of the 

 blood-stream are numerous, the most important being traumatism 

 (by injuring the vessels going to the region), endarteritis, throm- 

 bosis, tight bandaging, etc. They need not be discussed at 



