IMMUNITY. 



CHAPTER I. 



INTRODUCTION. DEFINITIONS OF IMMUNITY AND ANTIBODY. THE LAW 

 OF SPECIFICITY. THE NECESSITY OF CONTROL TESTS. 



The diagnosis of infectious diseases can be approached in several ways. 

 In addition to the aid obtained from clinical signs such as the course of the 

 temperature, the changes in the various organs, the exanthemata, etc., the 

 finding of the specific etiological agent of the disease, or the specific anti- 

 bodies developed by the reaction of the organism are of equal or even 

 greater importance. The course of an infection depends not only upon the 

 nature, the number, and the virulence of the infecting agents, but also upon 

 the behavior of the infected body. One must consider a disease as the result 

 of the interaction of both of these factors without necessarily being able to attrib- 

 ute the various symptoms to either the one or the other. Although the general 

 reaction of the organism is varied, it can nevertheless be shown that in 

 spite of even individual differences, the characteristic bacteria and their 

 products bring about a distinct symptom-complex which is usually con- 

 comitant with a significant defence on the part of the organism. The means 

 which the body employs in this protection are cellular and humoral in nature. 

 In fact, there is a group of infectious diseases in which the cellular reaction 

 predominates, and another in which humoral changes are pre-eminent; 

 and between these extremes are various intermediate forms. Thus the 

 constantly changing picture of tuberculosis always shows the tubercle as its 

 typical product of cellular reaction; similarly leprosy and syphilis have 

 their peculiar cellular changes. More difficult, however, to recognize by 

 the unaided eye or even the microscope are the finer biological alterations 

 which take place in the body fluids during the course of infectious diseases. 

 Here, special methods are necessary to detect and differentiate the various 

 humoral changes which occur for the main part, in the blood serum. As is 

 known at present, the humoral as well as the cellular immunity reactions 

 are not limited to infectious diseases, but also 'express normal physiological 

 and pathological conditions. With the conception of Ehrlich's side-chain 

 theory the bridge of understanding for the humoral reaction was built, and 



