INTRODUCTORY AND GENERAL 9 



certain regions) a decidedly rare cause of abscesses and other 

 pyogenic processes. It is evident that the form of immunity 

 which prevents the pneumococcus from gaining access to the 

 tissues and giving rise to abscess formation is in abeyance in the 

 young and well developed in the adult ; yet the two are more 

 nearly equal in their resistance to this organism in its role of a 

 producer of pneumonia. There are also very marked differences 

 in regard to local immunity in the two ages, but of those we shall 

 speak subsequently. 



Natural immunity must not be regarded as a fixed and definite 

 quantity, since all individuals vary enormously in their resisting 

 powers against various diseases at different times and under 

 different conditions. The factors which tend to break down the 

 immunity against any or all infections may be referred to as the 

 banal causes of the diseases in question. They are not in them- 

 selves sufficient to lead to these diseases, but when they come 

 into action and an infecting agent is present the disease will arise. 

 Hence they are often referred to as predisposing causes of disease, 

 and to the lay public they are the actual causes, since they are 

 usually open and obvious, and the real infecting agent is, of 

 course, unknown They are of the utmost importance in pre- 

 ventive medicine, and wherever the probability of an infection is 

 apprehended, a study of the patient's surroundings and habits 

 may often lead to the giving of advice by which these banal 

 causes of infection may be avoided and the disease warded off. 

 In general these predisposing causes are a study for the physician 

 rather than for the pathologist, and in some cases we are quite in 

 the dark as to the method in which they act. Their study cannot 

 be conveniently undertaken here before the mechanisms and pro- 

 cesses of immunity have been described, but it will be useful to 

 enumerate some of the more important. 



Of these cold and wet, especially in combination, are unquestion- 

 ably the most important. The exact way in which they act is 

 not definitely known, but there are materials for a number of 

 suggestions. Thus, as we shall have abundant opportunity of 

 seeing, immunity is to a very large extent a function of the leuco- 

 cytes, to which are specialized cells to which the defence of the 

 body is entrusted. Now the functions (movement and phagocy- 

 tosis) which can be easily investigated are found to be dependent 

 in a very high degree on temperature, acting best at the tempera- 

 ture of the body, or slightly above ; and it is highly probable that 



