PRESIDENTS ADDRESS — SECTION L. 273 



fail to give the reaction, but, significant fact, the serum of one of 

 the higher apes will react similarly to that of man, although not 

 ill so high a titre. The bearing of this specific reaction on 

 biological problems concerned with species relationships is pro- 

 found. 



Where bacteria are used as antigens the specificity of the antigen- 

 antibody reaction is often less clearly defined, for the reason that 

 the specific differences between the specieis and strains of bacteria 

 are less fixed. Thus typhoid, paratyphoid, and colon bacilli may 

 all be agglutinated by the same sample of human serum, but, 

 although showing this group relationship, if the human serum is 

 progressively diluted it will be found to give a reaction in much 

 higher dilution with the bacillus which is at the time infecting the 

 patient, i.e., the true antigen, so that it is quantitatively specific 

 even here. On thei other hand, the ability of two clift'erent micro- 

 organisms to produce agglutination, precipitation, or complement 

 fixation in the same serum suggests some family relationship 

 between the bacteria which is often of importance. A remarkable 

 instance of this, pointed out by an American worker, Miss Alice 

 C. Evansi (1918), and confirmed by myself, is that bacillus meli- 

 tensis and Bang's abortion bacillus will each agglutinate a bovine 

 serum from a case of abortion to the same high dilution. The 

 inability to differentiate between Johne's disease and tuberculosis 

 of cattle by means of a tuberculin reaction is similarly significant. 



We need not delay to discuss in detail the origin of the various 

 antibodies, but just as Metchnikoff's name will always be associ- 

 ated with phagocytosis, so will the name of Ehrlich be associated 

 with cur understanding cf the production of immune bodies in the 

 serum. 



Ehrlich's theory for the fcrmation of immune bodies may be 

 stated in its simplest terms in relation to the production of mifi- 

 fo.rin by the repeated and graduated injection cf to.rin. 



He conceived the body cells as depending for their nutrition on 

 the possession of certain affinities, or I'eceptors, able to combine 

 with food molecules (having similar receptors, or side-chains) pre- 

 sented to them in the blood. But, further, antigens, whether 

 bacteria or cells or foreign protein .molecules of any kind, were 

 conceived as having similar side-chains with affinities which could 

 also fit these cell receptors, and so the antigens become attached 

 to their appropriate cells. A cell so burdened with an antigen is 

 in danger of starvation, having insufficient free unsaturated recep- 

 tors wherewith to fix food molecules. In defence the cell proceeds 

 to bud off new receptors, and once this proliferation has com- 

 menced nothing operates to cause its cessation, and an over- 

 production of receptors takes place. Not cnly so, but these excess 

 receptors are set free into the blood stream, where, meeting with 

 analogous antigen (in this case, toxin) groups, they combine with 

 frhem, so satisfying the affinity of the toxin, making it incapable 



