GO 



tropins, and so forth, many other kinds of antibodies remain to 

 be discovered. This does not necessarily imply the existence 

 of an indefinitely large number of separate substances; some, 

 at least, of the antibodies which differ from each other may be 

 thought to be merely different properties of the same substance. 



Still, though the antigen-antibody conception of immunity 

 (as being concerned with the union of two substances possessing 

 chemical affinities) will doubtless lead to further discoveries of 

 importance, there are reasons, emerging from the biochemical 

 aspects of immunity, which indicate, to my mind, that demonstra- 

 tions of antigens and antibodies are only one aspect of immunity 

 phenomena. Such phenomena are essentially interactions be- 

 tween protein colloids, not between parcels of antigens and parcels 

 of antibodies. These colloids, it is true, may behave in some 

 respects as antigens and antibodies; but that is only part of 

 their behaviour, perhaps not always the most important part, 

 and certainly not the sum and substance of their chemico- 

 physical interactions. Therefore it seems desirable to guard 

 against making the assumption, perhaps unconsciously, that the 

 discovery of fresh reactions between antigen and antibody is 

 the only way, within the present limitations of knowledge, in 

 which further light can be thrown on immunity. Immunity 

 problems are not limited to the discovery of the right antigen 

 for the right antibody. 



In the first place, it is customary and proper to introduce a 

 discussion on immunity with the admission that very little is 

 known about it. All the main facts depend upon the vitality 

 of the animal body, the mechanism of which is too complicated 

 to be explained by known methods of scientific analysis. Imme- 

 diately after death, and before gross chemical or physical changes 

 have occurred, the body loses its capacity to resist bacterial 

 invasion, the reason evidently being that this capacity is 

 associated with extremely labile activities which become inert 

 as soon as the supply of oxygen and other essentials for vitality 

 is cut off. Vital substances cannot be preserved in their active 

 state for examination and analysis in the laboratory. Observa- 

 tions may be made on the properties of fresh blood, plasma, 

 serum, or leucocytes; but the most that can be claimed for 

 such experiments is that they may represent a faint and very 

 imperfect reflection of vital phenomena; it has to be adm tted 

 that they give very little information about what actually takes 

 place in the animal body, though of course, they may afford 

 evidence that the animal body has acquired new vital properties. 

 The facts termed vital resistance comprise a big and almost 

 unexplored territory, which may serve as a reminder that it is 

 unsafe to base an imposing structure of theory on conclusions 

 which travel beyond the facts. For this reason it is doubtful 

 whether the search for antigens and antibodies which are at 

 present unknown is all that is needed to provide a more solid 

 basis for this structure. 



