61 



Another limitation to the antigen conception arises from 

 the fact that one requirement of an antigen is that, in order to 

 stimulate the production of an antibody, it must possess some- 

 thing (see p. 54) which is " foreign " to the immunised animal. 

 Foreign protein stimulates the animal body to produce certain 

 antibodies such as precipitins, agglutinins, bacteriotropins, or 

 antitoxins ; and the methods of production, if not the actual 

 substances produced, are probably very much the same in each 

 case, the differences being mainly attributable to the antigens. 

 But the reactions of the animal body towards dead, or non- 

 cellular, foreign protein, though complicated enough, are much 

 less complex than the reactions which occur during the temporary 

 symbiosis between living parasitic bacteria and their animal 

 host. The living bacterial cell is much more than an antigen 

 ready for the reception of antibodies ; and its postulated 

 " antigen " is not fixed but is always undergoing metabolic 

 changes, which depend upon all the constituents of the bacterial 

 cell and upon its interactions with the tissues and fluids of the 

 animal host. Moreover, though dead bacteria are foreign protein, 

 a bacterial parasite, when thriving within the animal body, is not 

 behaving like foreign protein; so it cannot be assumed that 

 necessarily the best way to inhibit its growth is to discover an 

 antibody to its dead and disintegrating remnants, or that this 

 is nature's way of acquiring immunity. 



Three of the commonest and most important facts about 

 natural immunity are : — (1) resistance to saprophytes; (2) resist- 

 ance, on the part of a susceptible animal, to a small dose of 

 parasitic bacteria; (3) resistance of an animal belonging to a 

 naturally immune species. It cannot be shown that serological 

 antibodies to bacterial antigens are the explanation of any of these 

 three kinds of immunity. 



In immunity acquired by vaccination there are two facts to 

 note : — (a) increased resistance, and (6) generally, though not 

 invariably, the appearance of a serological antibody to the 

 injected protein. The second fact is undoubtedly associated 

 with the first, but can it be assumed that (6) is the complete 

 explanation of (a) ? It is hardly reasonable to think that there 

 is a sharp gulf between acquired immunity and types of natural 

 immunity such as (1), (2) and (3) ; I would prefer to assume that 

 in the former, as in the latter, factors other than antibodies 

 are of importance. Similar considerations may apply to passive 

 immunity. Many bacterial infections, notably anthrax, fowl 

 cholera, and swine erysipelas possess two features in common ; 

 their specific immune sera are not bactericidal in vitro and 

 attempts to explain the immunity by demonstrating specific 

 antibodies have failed to gain general acceptance. Quite 

 possibly the reason may be that the enhanced resistance of the 

 immunised animal is not due, or not wholly due, to the direct 

 or indirect action of an antibody on bacterial antigen. 



It has been suggested above that, in some cases, there may 

 be no demonstrable antigen-antibody reaction in association with 



