SIDE-CHAIN THEORY 91 



responsible for the agglutination of red corpuscles. This is a 

 phenomenon going hand in hand with hemolysis, but due to 

 an entirely different agency. Bordet first observed that the 

 serum of an animal which has been gradually injected with 

 the blood of a second has the power of producing an immediate 

 clumping of the corpuscles of the second animal when added 

 to a portion of the latter 's blood. The receptor which pro- 

 duces the clumping is supposed to be furnished at one end 

 with a functionating or agglutinophoric group, the other end 

 being a haptophoric group. When agglutination occurs, the 

 latter is supposed to be anchored to a suitable receptor of 

 the erythrocyte, so that the agglutinophoric group is enabled 

 to produce some unknown change in the cells which causes 

 them to clump together. The essential difference between 

 receptors of the third order and those of the second order is 

 that, in the former, the complementophilic group is replaced 

 by a zymophoric or acting group, which does not require to 

 become combined with complements before it can act. 



Not only toxins and erythrocytes, but also proteins, can, 

 when injected into an animal, lead to the production of specific 

 bodies with which they react. When proteins are injected 

 the reaction leads to a precipitation of the protein, and the 

 bodies responsible for this are known as precipitins. The 

 mechanism involved in the production of precipitins is identical 

 with that involved in the production of agglutinins, i. e., 

 receptors of the second class are regenerated in overexcess. 



According to the well-known theory of Metchnikoff, leuko- 

 cytes, as a result of their power of phagocytosis, ingest invad- 

 ing bacteria and render them innocuous unless the bacteria 

 be of such virulence as to poison the leukocytes. It has recently 

 been discovered that there are certain substances in normal 

 serum which act upon bacteria in such a way as to render 

 them susceptible of being taken up by leukocytes. These 

 substances are known as opsonins. By heating normal serum 

 to 65 C. for fifteen minutes its opsonin is destroyed and it 

 is then no longer capable of sensitizing bacteria toward leuko- 

 cytes. The opsonin probably possesses a haptophoric group, 

 by means of which it unites with the bacterium, and an activat- 

 ing group, whereby it produces some chemical or other change 

 in the leukocyte which stimulates the latter to absorb the 



