AmiEarch*  mr™' }    Bacterial  versus  Vegetable  Toxins.  129 
and  a  half  million  guinea-pigs,  while  the  lethal  dose  for  man  is  sup- 
posed to  be  0.30  Gm.  by  the  mouth  or  0.003  Gm.  subcutaneously. 
It  should,  of  course,  be  remembered  that  these  figures  do  not  refer  to 
pure  ricin,  but  to  ricin  contaminated  with  more  or  less  albumin. 
Smaller  doses,  injected  hypodermically,  soon  produce  immunity, 
antiricin  being  formed. 
As  previously  stated,  the  observation  of  Ehrlich  of  this  protective 
reaction  laid  the  foundation  of  serum  therapeutics.  Before  discussing 
the  production  of  antiricin  it  may  be  as  well  to  briefly  consider  the 
toxins  elaborated  by  bacteria.  When  pathogenic  bacteria  grow  and 
multiply  in  the  body,  symptoms  of  poisoning  (toxaemia)  are  mani- 
fested;  consequently  it  was  inferred  that  bacteria  either  produced 
poisons  during  their  growth  or  contained  poisons.  Subsequent  ex- 
periments proved  that  the  poisonous  effects  of  a  few  bacteria  which 
passed  out  into  the  surrounding  media,  and  the  poisonous  effects 
of  the  other  class,  seemed  to  be  due  to  the  actual  constituents  of  the 
bacterial  cells.  The  former  are  termed  exotoxins  or  extracellular 
toxins,  and  the  latter  endotoxins  or  intracellular  toxins.  Some  bac- 
teria, on  the  other  hand,  notably  the  tubercle  bacillus,  elaborate  both 
exotoxins  and  endotoxins.  These  toxins  are  substances  of  a  very 
complex  composition,  probably  allied  to  the  proteins.  In  some  in- 
stances they  appear  to  be  of  the  nature  of  enzymes  or  ferments,  and 
they  are  direct  products  of  the  bacterial  cells. 
Among  the  few  bacteria  producing  extracellular  toxins  or  exo- 
toxins the  bacillus  of  diphtheria  and  the  bacillus  of  tetanus  are  the 
most  important.  They  are  extremely  poisonous ;  in  fact,  it  has  been 
computed  that  the  toxin  elaborated  by  a  virulent  culture  of  the 
bacillus  tetanus  is  the  most  potent  poison  known  to  science  to-day. 
These  exotoxins  being  excreted  by  the  bacteria,  are  found  in  solution 
in  the  liquid  culture  media  containing  the  growing  microorganisms. 
Serum  from  an  animal  immunized  by  injecting  into  its  body  small 
and  increasing  doses  of  these  soluble  toxins,  administered  at  regular 
intervals  and  given  over  a  long  period  of  time,  is  rich  in  antitoxin. 
This  antitoxin,  when  properly  prepared  and  standardized,  constitutes 
the  antitoxin  of  commerce.  Diphtheria  and  tetanus  antitoxins  are 
prepared  in  this  manner. 
In  the  case  of  the  bacteria  which  do  not  secrete  a  soluble  toxin, 
the  killed  bacteria  themselves  are  used  for  immunization.  They 
constitute  what  are  known  as  the  antimicrobial  serums,  a  familiar 
example  of  which  is  antistreptococcus  serum,  and  should  be  distin- 
