IMMUNITY. 75 



injured they set free their digestive ferments, known as micro- 

 cytases and macrocytases respectively, which correspond to the 

 alexins of Ehrlich. 



Ehrlich's Side-chain Theory, This derives its name from the 

 fact that it presents an analogy to what happens in the benzol 

 ring of organic chemistry when its replaceable atoms of hydro- 

 gen are substituted by "side chains" of more or less complex 

 nature. The molecule of protoplasm is supposed to consist of 

 a central atom group provided with a large number of side 

 chains which subserve the vital processes of the molecule by 

 combining with other organic molecules. These side chains 

 are called receptors and are of many different kinds so as to fit 

 them for combination with many different varieties of extrane- 

 ous groups. Bacterial toxins contain two groups : (1) the 

 haptophores, by which the toxin molecule can become joined 

 to the cell, and (2) the toxophores, by virtue of which it can 

 attack the protoplasm after having been fixed to it by the hapto- 

 phore. If the attack on the molecule is not too severe, this is 

 stimulated into overactivity and throws out an abnormal num- 

 ber of receptors, some of which (the haptins) become detached 

 and are capable of uniting with free haptophores and prevent- 

 ing their combination with the protoplasm of the molecule. 

 In other words, they represent the antitoxin. 



Bacteriolysis is the destruction of the bacterial cells by the 

 blood-serum, and is probably effected in a somewhat different 

 manner. Antibacterial sera are effective through the combined 

 activities of a destructive element, the " complement" (alexin 

 or cytase), and an "immune body" (amboceptor) which serves 

 the function of joining the complement to the bacterial mole- 

 cule. These two bodies differ markedly in their properties for 

 example, the complement is destroyed at 60 C., while the 

 immune body is very resistant. 



It is not stated what cells are the sources of these various 

 anti-bodies, but probably any cell capable of being attacked by 

 a toxin is also capable of responding by the production of anti- 

 substances. 



Lysins. The substances producing destruction of bacteria 

 are called lysins. Normal blood-serum is bacteriolytic to a 



