^ecimbe^iso™'}     Opsonic  Theory  and  Bacterial  Vaccines.  561 
Wright  has  shown  that  the  opsonic  index  can  be  raised  if  we  inject 
beneath  the  healthy  skin  a  certain  number  of  killed  bacteria  of  the 
same  variety  as  those  which  are  infecting  the  patient.  The  number 
of  killed  bacteria  to  be  injected  and  the  intervals  between  doses 
have  been  carefully  studied  by  Wright  and  his  co-workers.  When 
the  proper  dose  of  killed  bacteria  has  been  injected  there  is  first  a 
fall  in  the  opsonic  index ;  this  is  followed  by  a  rise  above  the  normal. 
The  fall  in  the  index  is  probably  due  to  the  neutralization  of  the 
opsonins  which  are  in  the  blood  by  the  injected  bacteria — this 
is  called  the  negative  phase.  The  subsequent  rise  is  called  the  posi- 
tive phase,  and  it  is  during  this  period  that  improvement  in  the 
patient's  condition  is  looked  for.  It  is  not  necessary  to  give  a 
second  dose  of  killed  bacteria  so  long  as  the  index  remains  above 
normal. 
These  suspensions  of  killed  bacteria  are  called  bacterial  vaccines. 
There  has  been  some  discussion  as  to  the  use  of  this  word  which 
heretofore  has  meant,  to  the  popular  mind,  at  least,  that  material 
used  for  the  purpose  of  conferring  immunity  to  smallpox.  Accord- 
ing to  the  etymology  of  the  word  it  is  truly  a  misnomer  employed 
in  this  general  sense,  but  custom  has  sanctioned  its  use. 
A  vaccine  is  any  material  intended  for  administration  with  the 
idea  of  stimulating  the  production  of  active  immunity  to  a  certain 
disease.  We  vaccinate  man  to  protect  him  against  smallpox,  cholera, 
bubonic  plague,  typhoid  fever  ;  we  vaccinate  animals  to  protect  them 
against  anthrax,  black-leg,  erysipelas,  rinderpest.  Even  plants  may 
be  vaccinated. 
The  difference  between  an  antitoxin  and  a  vaccine  is  that  an  anti- 
toxin confers  passive  immunity,  while  a  vaccine  stimulates  active 
immunity.  The  vaccines  used  to  raise  the  opsonic  index  and 
thereby  bring  about  improvement  in  localized  infections  are  merely 
bacteria  suspended  in  normal  saline  solution. 
For  the  preparation  of  these  vaccines,  cultures  are  made  upon 
the  surface  of  a  solid  culture  medium,  usually  agar-agar.  These 
are  kept  at  the  body  temperature  for  about  twenty-four  hours  and 
the  resulting  growth  washed  off  with  sterile  salt  solution.  The 
thick  suspension  is  collected  in  a  proper  vessel  and  the  bacteria  are 
killed  at  6o°  C.  This  temperature  is  just  sufficient  to  destroy  the 
life  of  the  germs  with  little  injury  to  their  specific  immunizing  prop- 
erties.   The  bacterial  suspension  is  now  shaken  for  an  hour  or  more 
