558  Opsonic  Theory  and  Bacterial  Vaccines.  {^ecJleu™' 
is  essentially  a  chemical  one.  He  believes  that  the  mechanism  of 
immunity  is  allied  to  the  phenomena  of  nutrition  of  the  body  cells. 
For  its  nutrition  each  body  cell  is  believed  to  be  endowed  with 
radicals  possessing  side  chains  or  bonds  of  affinity ;  of  this  the 
benzol  ring  is  the  classic  illustration.  The  formula  for  benzol  is  C6H6. 
Written  out  graphically  we  find  each  atom  with  its  bonds  of  affinity 
perfectly  satisfied.  With  certain  chemicals,  however,  one  of  these 
hydrogen  atoms  may  be  split  off  and  other  elements  linked  to 
the  radical.  The  changes  which  take  place  in  the  building  up 
of  acetanilid  amply  illustrate  the  variety  of  substances  physically 
different  which  may  exist  without  the  slightest  change  in  the 
benzol  ring. 
H  H  H 
I  I  I 
c  c  c 
^   N  //   \  /X 
H— C        C— H  +  HN03=  H— C        C— N02+H4  =  H— C        C — NH4 
I  II  I  II  II: 
H— C        C— H  H— C        C— H  H    C        C— H 
X   /  %    /  %  / 
C  C  H 
•  1  J 
H  H  C 
Benzol.  Nitro-Benzol.  Anilin. 
H 
I 
C 
S  \ 
4-  CHoCOOH    =   H— C        C— NHoOOC— CH3 
I  II 
Acetic  Acid.  H— C        C— H 
H 
Amido-aceto-beuzol. 
The  number  of  different  molecular  groups  which  may  link  them- 
selves to  this  side  chain  reaches  almost  to  infinity. 
The  analogy  between  this  view  and  that  of  Ehrlich  concerning 
cellular  nutrition  and  the  production  of  acquired  immunity  is  very 
close.  The  cell  is  believed  to  select  its  nourishment  by  means  of 
side  chains,  but  instead  of  being  chemical  they  are  protoplasmic 
side  chains.  These  protoplasmic  side  chains  are  called  receptors. 
Now,  cells  have  receptors  not  only  for  substances  which  are  bene- 
ficial to  them  but  also  for  substances  which  tend  to  destroy  them. 
It  is  through  these  receptors  that  bacteria  by  means  of  their  toxins 
