2/8 PHYSIOLOGICAL CHEMISTRY. 



immunity is this formation of side chains in excess by over- 

 compensation, and is founded on the somewhat earlier Weigert 

 doctrine of cell regeneration and over-compensation in general. 

 Ehrlich has added the chemical conceptions of side chain 

 groups and has drawn numerous illustrations from organic 

 chemistry to show how they may act. Large molecules hold- 

 ing amino, sulphonic acid or halogen addition groups, for ex- 

 ample, may lose these or take them on again or take others 

 like them without losing their identity. Reagents acting on 

 such a large molecule attack, not the nucleus but these side 

 chains in general. The simple organic molecule has not the 

 power of self regeneration,, but the cell, which is a collection 

 of many such molecules, has trie power of forming new ma- 

 terials from the nutritive substances furnished to it. If whole 

 new cells are formed why not parts of cells or the outlying 

 side groups as well, and this is the Ehrlich assumption, which 

 is not unreasonable. 



As explained above these side chains or receptors are of 

 various kinds. Three distinct types or orders are easily rec- 

 ognized. Receptors of the First Order have one haptophor- 

 ous group and form antitoxins. That is, they combine chem- 

 ically with the soluble toxins in the serum and in a sense 

 neutralize them. Receptors of the Second Order have one 

 haptophorous group with which a foreign molecule or group 

 may be held and one special group which performs the func- 

 tion of an agglutinin or precipitin. Receptors of the Third 

 Order or amboceptors have two haptophorous groups with 

 which two things may be united. One of these is the foreign 

 cell (through its corresponding haptophorous group) and the 

 other the complement. In this way the complement or alexin 

 is able to work on the invading cell and attack it through its 

 "zymotoxic" group. These amboceptors are in themselves in- 

 active and can behave as cytotoxins (hemolysins or bacterioly- 

 sins) only when joined to the complement or ferment group. 

 They are formed in the serum by immunization with foreign 

 cells, and in turn combine with cells. 



