AnaMJa°yUriyi4arm'}       ^sc  °f  Antitoxins  and  Vaccines.  199 
quantity — their  effect  is  brought  about  by  a  much  more  complicated 
process.  We  believe  the  production  of  disease  by  this  class  of 
bacteria  is  not  a  function  in  which  they  alone  participate,  but  is  the 
result  of  their  interaction  with  the  body  cells. 
Antitoxins. 
Belonging  to  the  first  class  of  bacteria,  the  only  organisms  of 
interest  to  us  are  the  diphtheria  bacillus  and  the  tetanus  bacillus. 
These  produce  soluble  and  diffusible  poisons — toxins;  and  spon- 
taneous recovery  from  these  diseases  depends  upon  the  generation 
by  the  tissues  of  a  substance  which  will  neutralize  the  toxins — 
antitoxins.  The  requisite  antitoxins  can  be  easily  produced  in 
animals  and  transferred  to  the  bodies  of  patients  by  administering 
the  blood  serum  of  the  treated  animals. 
For  obvious  reasons,  horses  are  generally  selected  for  the  pro- 
duction of  antitoxin.  The  germs  in  question  are  developed  upon  a 
fluid  artificial  culture  medium — veal  broth.  After  the  bacteria  are 
removed  from  the  full-grown  culture,  the  sterile  filtrate,  containing 
the  specific  toxins,  is  injected  subcutaneously  into  the  horses.  The 
horses  react  by  the  production  of  antitoxin.  Enormous  quantities 
of  toxin  are  administered,  and  consequently  enormous  quantities  of 
antitoxin  are  generated  and  stored  in  the  blood  serum  of  the 
animal.  The  antitoxins  on  the  market,  then,  consist  of  this  blood 
serum,  either  native  or  chemically  treated  so  that  the  pseudoglobulin 
constituent  of  horse  serum  which  carries  with  it  the  antitoxic 
principle  is  removed  and  furnished,  in  solution,  in  as  pure  a  state 
as  possible. 
The  strength  of  the  antitoxin  is  determined  by  titrating  it  against 
toxin,  the  guinea  pig  being  used  as  indicator.  In  defining  a  unit  at 
present,  there  is  no  more  reason  to  say  that  it  is  the  amount  that 
will  neutralize  200  fatal  guinea-pig  doses  of  a  theoretically  pure 
toxin  than  there  is,  in  defining  an  inch,  to  say  that  it  has  a  definite 
relation  to  the  circumference  of  the  earth.  Twelve  inches  make  1 
foot,  36  inches  1  yard ;  1000  units  of  diphtheria  antitoxin  constitute 
the  immunizing  dose,  5000  units  the  average  initial  curative  dose ; 
1500  units  is  the  official  immunizing  dose  of  tetanus  antitoxin. 
The  action  of  diphtheria  antitoxin  may  be  clearly  illustrated 
by  imagining  the  disease  to  be  due  to  a  mineral  acid  generated 
within  the  body  and  poured  into  the  circulation  in  constantly  in- 
creasing quantities.    According  to  the  urgency  of  the  case,  let  us 
