$60  Opsonic  Theory  and  Bacterial  Vaccines.     { ^ecimber^goT^' 
(3)  Blood  serum. 
The  last  is  the  only  variant. 
Equal  parts  of  these  are  drawn  into  a  capillary  pipette  ;  after 
thorough  mixing,  the  pipette  is  sealed  and  the  mixture  is  kept  at 
the  body  temperature  for  about  fifteen  minutes.  During  this  time  the 
opsonins  which  are  contained  in  the  blood  serum  act  upon  the 
bacteria — opsonize  them.  Those  bacteria  which  have  been  prepared 
are  then  ingested  by  the  leucocytes.  After  incubation  the  contents 
of  the  pipette  are  again  thoroughly  mixed  and  spread  upon  slides. 
The  slides  are  properly  stained  and  examined  under  the  microscope. 
One  hundred  leucocytes  are  counted  and  the  number  of  bacteria 
they  have  ingested  is  recorded.  Two  different  mixtures  are  made, 
one  using  the  blood  serum  of  the  patient,  the  other  the  blood  serum 
from  at  least  one  healthy  individual.  The  opsonic  index  of  the 
healthy  person  being  taken  as  1,  the  proportion  between  the 
number  of  bacteria  ingested  in  the  two  cases  gives  us  the  opsonic 
index  of  the  patient.  For  example:  with  healthy  serum  one  hun- 
dred leucocytes  ingest  400  bacteria,  with  the  patient's  serum  the 
same  number  of  leucocytes  ingest  only  300  bacteria,  then  400 :  300  : 
1  :  X.    The  opsonic  index  of  the  patient  is  075. 
If  the  patient's  serum  prepared  500  bacteria,  his  index  would  be 
(400 :  500  :  1 :  X) — 125. 
One  of  the  important  practical  features  of  a  knowledge  of  the 
opsonic  index  is  its  value  as  an  aid  in  diagnosis. 
When  a  person  is  infected  with  a  certain  organism,  the  tubercle 
bacillus,  the  staphylococcus  or  whatever  it  may  be,  his  opsonic  index 
to  that  particular  organism  is  likely  to  be  below  normal,  it  may 
be  above,  or  the  equilibrium  may  not  be  disturbed. 
If  a  patient  is  suffering  with  some  obscure  disease  of  the  throat, 
for  instance,  and  it  is  impossible  for  the  laryngologist  to  decide 
between  cancer  or  tuberculosis,  a  tuberculo-opsonic  index  of,  say, 
0-75  is  almost  conclusive  evidence  of  tuberculosis.  The  diagnostic 
value  of  this  procedure  in  obscure  surgical  diseases  cannot  be  over- 
estimated. 
But  the  most  important  field  the  opsonic  index  has  occupied  is  in 
control  of  dosage  for  vaccine  therapy.  When  the  opsonic  index  is 
persistently  low  to  a  certain  organism  it  shows  us  that  the  resistance 
of  the  patient  is  low  to  infection  by  this  organism,  and  unless  his 
resistance  can  be  raised  in  someway  we  can  expect  no  improvement. 
