350 LIFE : OUTLINES OF GENERAL BIOLOGY 



it is not soluble but in the form of visible particles, these will be 

 agglutinated together in heavy clumps; this may also be the fate of 

 foreign cells, bacteria, for example, but cells or solid particles may 

 also be dissolved by the anti-body in the process called lysis. In the 

 case of lysis, however, the anti-body in the serum does not act 

 alone, but requires also the presence of an unknown substance 

 already present in serum called alexin or complement. Lastly, the 

 case in which the antigen is a poison {toxin), which is by no means 

 necessarily the case, must be considered; here the effect of the 

 anti-body (anti-toxin) is to neutralise the poison. 



It is possible that the anti-toxins are rather different in their 

 nature from other anti-bodies; but as the chemical nature of all 

 these substances is as yet unknown — they have never been obtained 

 in anything approaching a pure state — it is difficult to make definite 

 statements. It is probable that anti-bodies are proteins, but even 

 this is not securely established; and the mode and site of their 

 formation are equally obscure. It should be noticed that in some 

 cases the blood may contain natural anti-bodies capable of reacting 

 with antigens which have never been introduced, whilst those formed 

 after the injection of the antigen (immunising) are called immune 

 anti-bodies. 



One of the most striking features of these reactions is their great 

 specificity. A rabbit may be immunised to human blood, so that 

 when the rabbit's serum is mixed with a minute quantity of human 

 serum it will give a precipitate ; but with serum from an anthropoid 

 ape it will give no precipitate, or only a slight one. In this way 

 proteins can be identified with greater certainty than by chemical 

 methods, and in extremely minute quantities: and since the speci- 

 ficity of the reactions is quantitative rather than qualitative, the 

 fact that two proteins, though not identical, are related, may also 

 be demonstrated; for it seems reasonable to suppose that proteins 

 differing in their "immunity" reactions must also differ chemically, 

 although it may be hard to establish the latter point in particular 

 cases. The important fact is that although we know so little of the 

 reacting substances, there seems little doubt that the reactions 

 themselves are exactly analogous to reactions familiar in colloid 

 chemistry, such as the precipitation of colloids by removing the 

 electric charge on the particles. This is true even in the more difficult 

 case of lysis, with its intervening "complement", which is apparently 

 a complex of proteins, occurring in all serums naturally, and not 

 sjxicific. That is to say, it is capable of playing its part in any lytic 

 reaction. 



As immunity has a familiar Natural History aspect as well as a 

 deep physiological interest, we may discuss it less technically from 

 the wider point of view. 



It is a well-known fact of natural history that a hedgehog may 



