466 IMMUNITY 



various toxins, ferments, molecules of tissue cells, bacteria, red 

 corpuscles, etc. They are all probably of proteid nature, though 

 their true constitution is not known, and none of them have 

 been obtained in a pure condition. Amongst the latter may 

 be placed the various poisons of known constitution, glucosides, 

 alkaloids, etc. We may .also state at present that the anti- 

 substance forms a chemical or physical union with the particular 

 substance which has led to its development, and shall discuss 

 the evidence for this later. Furthermore, the anti-substance has 

 apparently a specific combining group which fits, as it were, a 

 group in the corresponding substance, the two groups having 

 been compared to a lock and key. It is, however, to be noted 

 that this specificity is a chemical one rather than a biological 

 one. An anti-serum, for example, developed by the injection of 

 bacterium A may also have some effect on bacterium B, and 

 thus appear not to be specific. We have, however, evidence to 

 show that some of the preponderating molecules in bacterium A 

 are also present in bacterium B, and thus the theory of chemical 

 specificity is not invalidated. The number of different anti- 

 substances, as judged by their combining properties, would 

 appear to be almost unlimited, a fact which throws new light 

 on the complexity of the structure of living matter. When anti- 

 substances are studied as regards their action on the substances 

 with which they combine, they may be conveniently arranged 

 in three classes corresponding to Ehrlich's three classes of 

 receptors (vide p. 491). In the first place, the anti-substance 

 may simply combine with the substance without, so far as we 

 know, producing any change in it, and to this group the anti- 

 toxins and anti-ferments belong. In the second place, the 

 anti-substance, in addition to combining, may produce some 

 recognisable physical alteration. In other words, it possesses 

 an active or zymotoxic group as well as a combining group. 

 The agglutiniris may be mentioned as examples of this group. 

 In the third place, the anti-substance after combination leads to 

 the combination of another body normally present in serum 

 called complement or alexine, and this latter, which has a con- 

 stitution very similar to that of a toxin, may lead to physical 

 change, for example, death or solution of a cell. Anti-substances 

 of this class are known as immune -bodies or amboceptors 

 (Ehrlich) or as sensitising substances substances sensibilisatrices 

 of French writers. Anti-substances of the second and third 

 groups are met with especially, though not exclusively, when 

 .f formed elements such as bacteria, red corpuscles, or tissue cells, 

 etc., are injected, the anti-serum developed possessing agglu- 



