CHAPTER XXVI 

 THE RELATION OF COLLOIDS AND LIPOIDS TO IMMUNITY 



WHILE at the present time Ehrlich's side-chain theory best explains 

 the specificity and mode of action of various antibodies, there is a grow- 

 ing tendency to explain many of these reactions on a physicochemical 

 and colloidal basis. 



From the fact that, without exception, antigens are colloids, and that 

 antibodies also are colloid in their chemical characters, it is advisable 

 to review briefly some of the main facts and theories concerning these 

 bodies and their reactions. 



Varieties of Colloids. Colloids may be composed of two different 

 classes of substances: 



1. Organic substances, as, e. g., all forms of proteins and also gums, 

 starch, glycogen, tannin, chondrin, the greater number of organic dyes, 

 and probably the enzymes. 



2. Inorganic substances, as, for example, the inorganic colloids, such 

 as silicic acid, ferric hydroxid, arsenic sulphid, and many other similar 

 compounds. 



Since the living tissues and fluids are, without exception, colloids and 

 colloidal solutions, the properties of the cells are largely the properties of 

 colloids. 



Nature and Properties of Colloids. Since Graham, in 1861, studied 

 the differences between the substances that did or did not diffuse readily 

 through animal or parchment membranes, soluble substances have been 

 classified in two main groups : (a) Colloids, or those substances that were 

 dissolved to the extent of showing no visible particles in suspension, but 

 that did not pass through diffusion membranes at all, or did so very 

 slowly indeed, and (6) crystalloids, or solutions that diffuse through 

 membranes quite readily. 



On the other hand, we may have substances that are quite insoluble 

 when aggregated in masses, but when derived in pure form by me- 

 chanical means, can be suspended and uniformly distributed through a 

 fluid without showing any marked tendency to precipitate. Such 

 suspensions or emulsions contain particles that are visible under the 

 microscope; they usually appear turbid, do not transmit electricity, and 



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