CH. XXIX.] THE SIDE-CHAIN THEORY 475 



the body ; passive immunity by the injection of a protective serum. 

 Of the two the former is the more permanent. 



Ricin, the poisonous protein of castor-oil seeds, and abrin, that of the 

 jequirity bean, also produce, when gradually given to animals, an im- 

 munity, due to the production of anti-ricin and anti-abrin respectively. 



Ehrlich's hypothesis to explain such facts is usually spoken of as 

 the side-chain theory of immunity. He considers that the toxins are 

 capable of uniting with the protoplasm of living cells by possessing 

 groups of atoms like those by which nutritive proteins are united to 

 cells during normal assimilation. He terms these haptophor groups, 

 and the groups to which these are attached in the cells he terms 

 receptor groups. The introduction of a toxin stimulates an excessive 

 production of receptors, which are finally thrown out into the circula- 

 tion, and the free circulating receptors constitute the antitoxin. The 

 comparison of the process to assimilation is justified by the fact that 

 non-toxic substances like milk or egg-white introduced gradually by 

 successive doses into the blood-stream cause the formation of anti- 

 substances capable of coagulating them. 



Up to this point I have spoken only of the blood, but month by 

 month workers are bringing forward evidence to show that other 

 cells of the body may by similar measures be rendered capable of 

 producing a corresponding protective mechanism. 



The substances which on injection provoke the appearance of 

 antidotes of this nature are of protein or protein-like nature ; they 

 are spoken of as antigens. 



One further development of the theory I must mention. At least 

 two different substances are necessary to render a serum bactericidal 

 or globulicidal. The bacterio-lysin or haemolysin consists of these 

 two substances. One of these is called the immune body, the other 

 the complement. We may illustrate the use of these terms by an 

 example. The repeated injection of the blood of one animal (e.g., the 

 goat) into the blood of another animal (e.g., a sheep) after a time 

 renders the latter animal immune to further injections, and at the 

 same time causes the production of a serum which dissolves readily 

 the red blood-corpuscles of the first animal. The sheep's serum is thus 

 hsemolytic towards goat's blood-corpuscles. This power is destroyed 

 by heating to 56 C. for half an hour, but returns when the fresh 

 serum of any animal is added. The specific immunising substance 

 formed in the sheep is called the immune body; the enzyme-like 

 substance destroyed by heat is the complement. The latter is not 

 specific, since it is furnished by the blood of non-immunised animals, 

 but it is nevertheless essential for haemolysis. Ehrlich believes that 

 the immune body has two side groups one which connects with the 

 receptor of the red corpuscles, and one which unites with the hapto- 

 phor group of the complement, and thus renders possible the enzyme- 



