106  Nature  and  Action  of  Toxins.  {AniiiX.imrm 
(b)  If  the  toxin  contains  only  T,  the  body  must  furnish  i ;  if  an 
antitoxin  is  produced,  this  must  consist  of 
(c)  If  the  toxin  contains  only  i,  the  body  must  furnish  T ;  the 
antitoxin  must  be 
R<T 
The  first  case  occurs  with  diphtheria  and  tetanus  toxins ;  these 
contain  both  the  T  and  the  i  molecules ;  the  last  occurs  with  snake 
venom,  which  contains  only  i.  The  second  case  occurs  with  most 
bacteria  which  do  not  form  soluble  toxins  in  cultures ;  as  also  with 
the  formation  of  precipitins,  etc.,  by  the  injection  of  living  foreign 
cells  or  substances. 
An  example  may  make  this  clearer:   The  milk  of  a  cow,  when  it 
is  injected  under  the  skin  of  a  rabbit,  acts  toward  the  biogen  of  the 
rabbit  as  if  it  contained  a  T-group. 
This  group,  combining  with  the 
R<' 
of  the  biogen,  stimulates  these  to  an  overproduction  of 
R<* 
molecules,  which  are  discharged  into  the  serum.  These  molecules 
have  the  power  of  combining  with  the  T-molecules  of  the  milk. 
This  fact  is  shown  by  the  production  of  a  precipitate  when  the 
serum  of  a  rabbit  thus  treated  is  added  to  a  sample  of  cow's  milk. 
This 
R<i 
is  therefore  called  a  "  precipitin."  The  serum  of  a  rabbit  which 
has  not  been  treated  to  milk  injections  does  not  produce  this  pre- 
cipitate. The  T  of  the  cow's  milk  being  different  from  the  T  of  any 
other  milk,  the  serum  of  a  rabbit  which  has  been  treated  with  cow's 
milk  will  not  precipitate,  e.g.,  goat's  milk.  Nor  will  one  treated 
with  goat's  milk  precipitate  cow's  milk.  Since  the  proteids  of  an 
animal  do  not  contain  any  molecules  which  act  as  T  toward  the  cells 
