AnMarch.f£7.rm'l     The  Opsonins  and  Bacterial  Vaccines.    .  143 
mococcic  sera  evidence  of  substances  which,  while  inactive  towards 
leucocytes,  possessed  very  marked  opsonic  or  sensitizing  action,  as 
they  termed  it,  toward  corresponding  cultures  of  streptococci  and 
pneumococci. 
Ross  {Lancet,  November,  1906)  summarizes  our  knowledge  of  the 
opsonins  as  follows : 
44  (1)  Opsonins  act  by  chemically  uniting  with  the  invading  bac- 
teria, and  so  altering  them  that  the  leucocytes  are  able  to  phagocyte 
the  bacteria  and  destroy  them.  It  is  important  to  remember  that 
these  substances  do  not  stimulate  or  otherwise  affect  the  leucocytes. 
"  (2)  It  is  probable  that  there  are  present  many  varieties  of  opso- 
nins in  the  blood  plasma,  each  having  to  do  with  combating  a  par- 
ticular kind  of  microbic  invasion. 
"  (3)  Opsonins  have  been  shown  to  be  distinct  from  other 
bacteriotropic  substances,  such  as  the  bacteriolysins,  the  agglutinins, 
and  the  antitoxins." 
The  leucocytes  of  healthy  or  diseased  persons  seem  to  be  equally 
active  when  brought  in  contact  with  the  same  serum,  hence  the 
amount  of  opsonins  present  in  the  blood  of  an  individual  determines, 
according  to  the  opsonic  theory,  his  susceptibility  to  bacterial 
invasion. 
TECHNIQUE. 
To  measure  the  resistance  of  the  patient  to  such  invasion,  or  to 
find  out  his  opsonic  index,  special  technique  has  been  developed, 
which  may  be  briefly  described  as  follows: 
I.  EMULSIONS  OF  BACTERIA. 
Twenty-four-hour  or  younger  growths  of  the  rapid-growing  bac- 
teria, as  Staphylococci,  Streptococci,  Pneumococci,  Gonococci  and 
Colon  bacilli,  upon  inclined  agar  are  washed  off  with  normal  saline 
solution.  After  the  mixture  has  sedimented,  the  upper,  whitish 
layer  composed  of  fluid  and  bacteria  is  removed  with  a  pipette,  and 
the  finer  clumps  of  bacteria  precipitated  by  placing  the  fluid  in  a 
rapidly  rotated  centrifuge  for  a  few  minutes.  The  supernatant  layer, 
which  is  still  opalescent  and  is  called  a  bacterial  emulsion,  should  if 
suitable  for  work  contain  the  germs  in  a  well- separated  condition. 
Cultures  of  tubercle  germs  are  heated,  and  ground  in  a  mortar 
with  salt  solution  until  the  mass  is  well  broken  up,  and  then  centrif- 
ugated.    In  case  glycerin  cultures  are  used,  such  as  are  left  in  the 
