Theories  of  Immunity.— Pasteur  believed  that  during  the  first  attack  of
a  disease  the  bacteria  consume  certain  tissue  elements  necessary  for  their
existence,  and  that,  therefore,  immunity  e.xisted  until  these  elements  had  been
renewed  (exhaustion  theory).  Chauveau  thought  the  excretions  of  bacteria
created  a  field  unfavorable  for  their  development  (retention  theory).  Metsch-
nikofT  explains  the  destruction  of  microbes  in  the  body  by  phagocytosis,  i.e..
microbes  are  devoured  and  digested  by  certain  cells  of  the  body,  especially

I  the  leukocytes;  these  cells  he  calls  phagocytes.  The  substances  existing  in
the  blood  scrum  which  prepare  bacteria  for  phagocytosis  are  termed  opsonins.
Buclmer  holds  that  the  bactericidal  power  of  the  blood  and  body  juices
is  due  to  certain  albuminous  bodies,  or  alexins  (humoral  theory).  It  has
been  sho\sTi  that  the  senim  of  animals  immunized  to  the  bacilli  of  typhoid
fever,  cholera,  and   the  bacillus  coli,  cause  agglutination,  or  clumping,  of


i


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ihe  respective  microbes.  The  NVidal  test  for  typhoid  fever  is  based  on
this  phenomenon;  some  believe  this  cliunping  to  be  a  preliminary  step  to
bacteriolysis  (dissolving  of  bacteria  by  alexins).  Ehrlich  explains  the  phe-
nomena of  immunity  by  the  side-clutin  theory.  He  believes  that  every
living  cell  consists  of  a  central  body,  and  of  a  number  of  other  chemical
groups  or  side-chains  (receptors)  which  are  especially  concerned  with  nutrition.
