CHAPTER XII 

 COLLOIDAL THEORY OF ANTIBODIES 



THE fact that antigens and their antibodies are, as far as is known 

 at present, exclusively colloid in chemical character renders it 

 advisable to glance briefly at some of the main facts and theories 

 concerning these bodies and their reactions. They are substances 

 of the greatest possible interest to the biologist, since the living 

 tissues and the fluids of the living animal are, without exception, 

 colloids and colloidal solutions ; protoplasm, cell nuclei, etc., being 

 mixtures of colloids in a jelly-like form, whilst the fluid part of 

 blood-lymph, etc., is a colloidal solution. It is necessary to 

 remember this, since the reactions of colloids and of crystalloids 

 in presence of colloid appear to follow laws which are different 

 from those governing the reactions of crystalloids alone, and we 

 may doubt whether these latter ever take place in the living body. 

 Colloids are not necessarily organic bodies, since metals such as 

 gold and silver may be obtained in the colloidal state, as well as 

 many metallic salts, such as ferric oxide, silicic acid, etc. These 

 simple colloids appear to be governed by laws similar to those 

 concerned in the reactions of the organic colloids formed by the 

 action of vital processes. 



The chief features which distinguish colloids from crystalloids 

 are (i) that they do not undergo dissociation into their ions when 

 dissolved in water ; and (2) that this so-called solution is not a true 

 one, but merely a suspension of unaltered molecules or groups of 

 molecules. These two characters are probably fundamentally the 

 same. A colloid solution, therefore, is merely a very fine emulsion 

 or suspension of particles of the substance, and Siedentopf and 

 Zsigmondy have demonstrated a method by which these particles, 

 though infinitely small as compared with the most minute bacteria, 

 can be rendered visible and their numbers estimated. The method 

 is simple enough in theory, though the actual arrangements are 

 somewhat complicated. A powerful beam of light is passed trans- 



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