THE DEFENSIVE MECHANISMS OP THE BLOOD. 151 



the fact that nervous tissue contains a chemical substance which 

 unites readily with the haptophore group of the tetanus toxin, 

 and also substances that are readily attacked by the toxophore 

 group of the toxin. The antitoxins are supposed to act by com- 

 bining with the haptophore group, thus preventing the toxin 

 from uniting with the cell. 



According to this theory the formation of antitoxins may be 

 accounted for as follows : When a receptor, as we may term the 

 portion of the cell which unites with haptophore group, is united 

 to the toxin, the cell endeavors to adapt itself to the loss of this 

 radicle by the production of another similar one. Since the gen- 

 eral rule of nature is to respond to an action with an over-reac- 

 tion, many more receptors are made than are actually needed to 

 unite with the haptophore groups of the toxin present. The re- 

 ceptors produced in such great number break away from the 

 parent cell. These accordingly are stored up in the blood, and 

 whenever any of the particular toxin for which they are adapted 

 is present in the circulation, they unite with it and thus prevent 

 the toxin from uniting with the tissue cells. A body which pos- 

 sesses a store of such antibodies is said therefore to be 

 immune. 



Toxins are not the only substances which will produce specific 

 antibodies. This property is a general characteristic of proteins. 

 Any substance producing an antibody is known as an antigen. 

 For example, if human blood be injected into a rabbit, and after 

 several days some of the rabbit's blood serum is mixed with hu- 

 man blood serum, a precipitate will form, whereas the blood of 

 a normal rabbit will produce no such precipitate. The first in- 

 jection of human blood serves to stimulate the rabbit cells to 

 form some substance which precipitates any human blood sub- 

 sequently added. The reaction is specific, for the blood of any 

 other species of animal will not be precipitated by blood from a 

 rabbit sensitized with human blood, and the reaction offers a very 

 accurate method of differentiating between human blood and 

 other blood in medico-legal cases. The body thus formed is known 

 as a precipitin. 



Anaphylaxis. Again, if a rabbit be injected with some hu- 



