IO2 CONDITIONS OF ANTIBODY PRODUCTION 



diphtheria toxin, etc., as cell pabulum is doubtful, but there are 

 some experiments which go to show that this is the case. Thus, 

 the rat is relatively immune to diphtheria toxin, and shows no 

 symptoms after the injection of an amount which is lethal for 

 many rabbits, and this immunity is not due to the presence of 

 antitoxin. This being so, it seems clear that the diphtheria toxin 

 disappears from the blood of the rat in virtue of forming a com- 

 bination with the cells of the latter without injuring them, and we 

 may fairly assume that it is of value in nutrition, although, of 

 course, this assumption is not open to direct proof. In a similar 

 way tetanus toxin disappears rapidly from the blood of scorpions, 

 no antibody being formed. 



Thus it appears to be a logical sequel of the acceptance of 

 Ehrlich's theory that we may define proteids as substances which, 

 when injected into suitable animals, give rise to the production of 

 antibodies. We have now to glance for a moment at the ques- 

 tion of the suitability of different animals for the production of 

 antibodies. 



Researches with precipitins show that the proteid substances 

 which are present in the serum of any species of animal are 

 different from those which occur in any other. We must regard 

 the molecule of proteid of any type (say of a globulin) as being of 

 great complexity, and capable of many slight modifications, which 

 are inappreciable to gross chemical tests, but which are perfectly 

 obvious to the living animal cells. Human globulin differs from 

 horse globulin, and this, again, from sheep globulin. Further, 

 some facts go to show that differences exist between the proteids 

 of animals of the same species, and that the proteids in the serum 

 of one man or horse are not exactly the same as those in another 

 man or horse. These differences come in as a result of the last 

 step in the process of digestion ; the food materials are broken 

 down into simpler bodies in the alimentary canal, and built up 

 again in their passage through the epithelial cells which line that 

 structure, and in this process receive certain distinctive features 

 peculiar to the species or to the animal in question. As a result, 

 the body fluids of any animal contain proteids which have been 

 deliberately adapted for the nutrition of the cells of that animal, 

 but which may be quite useless or even toxic for those of another 

 species. The prime requisite of their suitability for nutritive 

 purposes is that the proteid molecules should possess haptophores 

 which " fit " the receptors of the cell molecules ; the second, 



