196 THE ANTIOEN-ANTIBODY REACTIONS 



recent years they are now rarely employed. It is usually better, for reasons that 

 are discussed in later sections of this chapter, to rely on the term antigen, allowing 

 the context to make clear which particular antigen or antigenic component is 

 indicated. 



These general rules of nomenclature have certain exceptions. When the 

 antigen itself possesses some well-marked biological activity, which is neutralized 

 by the antibody, the antibody is named by adding the prefix anti- to the name 

 of the antigen. Thus, the antibody which neutralizes a toxin is called an anti- 

 toxin, the antibody which neutralizes snake-venom is called antivenin, and so 

 on. In many cases descriptive terms have been retained for the designation of 

 antibodies, instead of coining new names, and this practice has much to recommend 

 it in the present state of our ignorance. 



It will be observed that the thermolabile substance, which is present in normal 

 serum and which appears to be the active agent in bringing about the lysis of 

 sensitized cells and the death of sensitized bacteria, demands a separate name. 

 This substance is not an antibody, since it is not increased in amount as the result 

 of immunization, and it must for the moment be regarded as in a class by itself. 

 Its original name of alexine has become very generally replaced by the name com- 

 plement, which was introduced by Ehrlich. It may be defined as follows. 



Complement {or alexine) is a thermolabile substance, or complex of substances, 

 present in varying concentrations in the blood serum of most normal animals, which 

 has the property of bringing about the lysis of certain cells and bacteria, in conjunction 

 with certain antibodies which render the cells or bacteria sensitive to its action. 



This definition is incomplete, for it begs the question as to whether complement, 

 acting by itself, produces any significant reaction whatever. It describes, 

 however, with sufl&cient accuracy, the part which complement plays in the par- 

 ticular reactions with which we are concerned. 



Theories of the Mechanism of the Serum Reactions. 



Before entering on a detailed discussion of the various serum reactions, it is 

 well to have a general idea of the theories which have been propounded to account 

 for the phenomena observed. 



One theory, which has played a prominent part in immunological studies, was 

 propounded by Ehrlich (1898, 1900). It has been developed and modified with 

 extreme ingenuity by himself and by his colleagues to meet the demands made 

 upon it by the continuous accumulation of new and often disconcerting facts, 

 which had to be fitted into a structure growing more and more complex, and 

 obviously becoming a little strained, as one new hypothesis after another was 

 added to the central conception. 



According to this " side-chain " theory we should regard the cell as being 

 built up of highly complex chemical aggregates, with attached groupings, or 

 side chains, the normal function of which is to anchor nutrient substances to the 

 cell, and in some cases to act upon and modify them, as a preUminary to their 

 incorporation into the essential cell substance. These side-chains, or receptors, 

 thus form the point of contact between the cell substance and any other materials 

 which gain access to the fluids in which the cell is bathed. It is only by gaining 

 attachment to these receptors that substances of the class to which antigens belong 

 can exert any action on the cell, and so stimulate it to activity. In Ehrlich's 

 view the antigens, whatever their nature, attach themselves to these cell-receptors. 

 Since the antigens are in all cases foreign substances, which have no part in the 



