ADecimber!hi907f'}     Opsonic  Theory  and  Bacterial  Vaccines.  557 
The  study  of  immunity,  the  foundation  of  which  was  laid  by 
Pasteur  and  Koch,  modest  in  its  beginnings,  has  grown  to  be,  in  my 
opinion,  the  most  important  of  all  sciences.  The  application  of 
principles  evolved  from  investigations  in  immunity  has  resulted  in 
the  lengthening  of  the  average  life  of  man  by  several  years,  and  we 
have  not  yet  begun  to  fathom  its  possibilities. 
Since  the  time  of  Pasteur  many  theories  have  been  evolved  to 
explain  the  mechanism  of  our  resistance  to  communicable  disease. 
Attempts  have  been  made  to  satisfy  our  curiosity  as  to  why  one  has 
scarlet  fever,  as  a  rule,  but  once  ;  why  a  single  attack  of  typhoid  fever 
endows  one  with  the  power  of  drinking  Schuylkill  water  with  impu- 
nity ;  why  a  successful  vaccination  protects  against  smallpox.  One  of 
the  early  hypotheses  is  that  known  as  the  "  Exhaustion  Theory."  Of 
this  the  following  illustration  has  been  given  :  a  man  coming  from 
market  with  a  number  of  different  kinds  of  fruit  in  his  basket  is 
attacked  by  several  boys.  One  boy  who  is  fond  of  no  fruit  but 
apples  eats  all  the  apples,  another  who  cares  only  for  peaches  eats  all 
the  peaches,  the  third  who  has  a  taste  for  bananas  alone  exhausts 
the  supply  of  this  fruit,  and  so  on.  In  other  words  during  an  attack 
of  typhoid  fever,  for  example,  some  substance  in  the  normal  body 
is  "  eaten  up  "  by  the  typhoid  bacillus,  and  the  individual  is  hence- 
forth immune  to  this  disease.  After  much  discussion  and  experi- 
mentation this  theory  of  Pasteur's  was  found  to  be  untenable. 
Next  the  "  Retention  Theory  "  attracted  much  notice.  This  is 
just  the  opposite  of  the  exhaustion  theory,  in  that,  instead  of  taking 
something  away  from  the  body,  certain  materials  were  supposed  to 
be  added  by  the  growth  of  infective  bacteria  in  the  tissues. 
The  two  theories  that  have  occupied  the  field  for  the  last  few 
years  are  the  side-chain  theory  of  Ehrlich  and  the  phagocytic  theory 
of  Metchnikoff.  According  to  the  latter,  bacterial  diseases  are 
resisted  by  the  leucocytes — the  white  blood  corpuscles.  When  one 
of  these  meets  a  bacterium,  it  encloses  and  ingests  it  and  may  kill 
it.  If  the  germ  is  very  strong  or  virulent  the  leucocyte  may  not  be 
able  to  kill  it,  but  may  itself  be  killed.  When  the  leucocytes  are 
unable  to  destroy  invading  organisms  an  infection  results.  The 
process  of  the  ingestion  of  bacteria  by  the  leucocytes  is  called 
phagocytosis,  and  the  white-blood  corpuscles  performing  this  func- 
tion are  called  phagocytes. 
The  side-chain  theory  of  Ehrlich  is  not  so  easy  of  explanation,  it 
