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 attribute 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 concomitant with a significant defense 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 tubercu- 

 losis 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 

 it at once became evident how the physiological phenomena of nutrition 

 and production of energy are identical in their nature with processes which 



