^ecimbe^K1"}     Opsonic  Theory  and  Bacterial  Vaccines.  559 
and  other  metabolic  products  attack  the  cell.  When  a  cell  has  been 
thus  set  upon  and  injured,  the  reparative  powers  of  the  body  exer- 
cise their  function  and  one  evidence  of  repair  is  the  production  of 
more  receptors.  As  nature  is  prodigal  in  all  her  reparative  pro- 
cesses an  excess  of  receptors  is  manufactured  and  some  are  crowded 
out  into  blood  and  tissue  juices.  A  large  number  of  these  receptors 
floating  in  the  serum  constitutes  the  antitoxins  and  antibacterial 
serums. 
There  are  as  many  different  kinds  of  receptors  as  there  are  sub- 
stances which  attack  the  cells.  The  haptophore  group  (as  it  is 
called),  of  the  toxin  or  poison  must  exactly  fit  the  receptor  or  there 
is  no  toxic  effect  produced.  The  mongoose  is  not  affected  by  a  snake 
bite,  in  other  words,  the  cells  of  the  mongoose  have  no  receptors 
which  fit  the  haptophore  group  of  snake  venom. 
This  brings  us  to  the  place  which  the  opsonic  theory  of  Wright 
occupies  in  the  study  of  immunity.  For  a  number  of  years  the 
French  school,  with  MetchnikofT  at  its  head,  has  been  maintaining 
that  the  formed  elements  of  the  body  are  responsible  for  resistance 
to  disease,  while  Ehrlich  and  the  German  school  contends  that  the 
body  fluids  are  of  most  importance  in  this  connection.  Wright  has 
shown  that  both  these  elements  are  almost  equally  important — the 
leucocytes  ingest  and  destroy  infecting  bacteria  but  are  able  to  do 
this  only  after  the  latter  have  been  acted  upon  by  some  substance 
in  the  serum  of  the  blood.  The  substances  in  the  blood  and  tissue 
juices  which  prepare  bacteria  for  phagocytosis  have  been  called 
opsonins — from  a  Greek  word  meaning  to  prepare  for  food.  Immunity 
according  to  this  theory  depends  upon  phagocytosis,  and  phagocy- 
tosis depends  upon  the  opsonins.  Obviously  then  the  degree  of 
immunity  depends  upon  the  opsonic  content  of  the  blood.  The 
ingenious  technique  devised  by  Wright  and  his  co-workers  has 
facilitated  the  estimation  of  these  substances. 
Definition.— -The  opsonic  index  expresses  the  power  of  the  blood 
serum  of  the  individual  under  examination  to  prepare  bacteria  for 
phagocytosis  compared  with  the  same  power  of  the  blood  serum  of 
a  healthy  individual  under  exactly  the  same  conditions. 
Technique. — To  determine  the  opsonic  index  three  things  are 
necessary : 
(1)  A  suspension  of  bacteria. 
(2)  A  suspension  of  healthy  leucocytes. 
