2.  To  determine  whether  or  not  a  nurse  or  attendant  is
immune  to  diphtheria.  A  non-immune  person  ought  not  to  be
allowed  to  care  for  a  diphtheria  case.
3.  To  determine  immunity  in  experimental  work.
The  Schick  test  shows  that  1000  units  of  antitoxin  produce
a  passive  immunity  that  lasts  about  thirty  days.  If  another
1000  units  are  given,  the  immunity  lasts  for  from  seven  to  ten
days  only,  for  the  dose  partly  sensitizes  the  body  to  horse-
senmi,  and  causes  it  to  throw  oflf  the  second  dose  more  quickly
than  it  did  the  first  one.
Toxin-antitoxin  Immunity. — ^An  active  immunity  to  diph-
theria may  be  induced  by  the  subcutaneous  injection  of  a  mix-
ture of  toxin  and  antitoxin.  The  standard  mixture  consists  of
toxin  amounting  to  200  fatal  doses  for  a  guinea-pig,  and  of  an-
titoxin in  sufficient  amount  to  ovemeutralize  the  toxin.  Three
injections  are  given  a  week  apart.  A  protective  amount  of  anti-
toxin appears  in  the  blood  in  about  a  month,  and  persists  for
years.  Over  90  per  cent,  of  persons  who  have  no  antitoxin  in
their  blood  will  produce  antitoxin  under  the  stimulus  of  the
toxin-antitoxin  mixture.
The  toxin-antitoxin  method  of  inducing  an  immunity  to
diphtheria  is  as  valuable  and  practical  as  the  injection  of  vac-


DIPHTHERIA  -221

i  in  the  prevention  of  typhoid  fever.    A  health  officer  will
e  it  in  producing  permanent  imniimity  in  persons  who  show
L  positive  reaction  to  the  Schick  test.
Carriers. — Diphtheria  germs  may  grow  in  the  throat  without
xoducing  sickness.  They  usually  disappear  from  the  throat
un  about  two  weeks  after  a  person  recovers  from  diphtheria.  If
tliey  persist  for  three  weeks  or  more,  the  person  is  qiassed  as  a
carrier.
