56 



immunity work, therefore, care has to be exercised in establishing 

 the quantities of the reagents which will secure an optimum 

 reaction. 



The importance of this point is well illustrated by " inhibition 

 zone " phenomena. 



2. Differences in the Mechanism of Reaction.— These may be 

 largely influenced by colloidal conditions. According to Land- 

 steiner, there are two general types of immunity reactions. 

 In the first, as exemplified by agglutination, precipitation, and 

 the union of toxin with antitoxin; there is the simple union of 

 two colloids. In the second, as exemplified by cytolysis, colloids 

 (or a complex colloidal mixture) bring about a destruction of 

 lipoid-protein combinations, or of cell-membranes which contain 

 lipoidal constituents, the result being the release of soluble 

 cellular contents. Another important fact is that the presence 

 of a third colloid, such as normal serum or even an inorganic 

 colloid, may profoundly influence the behaviour of antigen and 

 antibody. 



Such circumstances as these show that the reactions of 

 antigens and antibodies cannot be expressed in terms of merely 

 chemical combination. 



3. Conception of a " Pure " Antigen.— From chemical con- 

 siderations alone, it has appeared necessary to postulate that 

 the special chemical groupings which characterise an antigen 

 can only exercise their functions when united to a protein 

 molecule, and that, therefore, it is impossible to effect a chemical 

 separation of the purely specific antigenic components from the 

 rest of the molecule, or to regard the protein as merely an 

 unessential attachment to the antigen. This difficulty, as 

 regards the individuality of an antigen in its chemical aspects, 

 becomes still greater when the essential colloidal attributes 

 of an antigen are considered. Whilst not disregarding the 

 importance of chemical constitution, it has to be remembered 

 that the participants in antigen-antibody reactions are large 

 colloidal complexes and are not substances possessing chemical 

 individuality in the ordinary sense in which this term would be 

 understood by the organic chemist. 



Hence a " pure " antigen is an abstract conception which is 

 very different from the concrete reality. 



4. End-Results depend on a Sequence of Reactions. — According 

 to the biochemical view, the specific activity of an antigen does 

 not depend merely on a particular grouping of atoms within the 

 molecule or on one special physico-chemical constant ; specificity 

 is the resultant of a long series of different phenomena which 

 follow one another in causal sequence, just as the unlocking of 

 one door gives access to a second, and the unlocking of the second 

 to a third. Immunity is particularly concerned with a special 

 class of these "linked" reactions, viz., with those processes 

 where an intermediate reaction is required to provide the energy 

 necessary for a further reaction, for example, when a must first 



