774 
Diphtheria  Antitoxin. 
Am.  Jour.  Pharm. 
Nov.,  1918. 
as  the  limit  of  concentration  is  reached  at  which  the  more  readily 
precipitated  protein  falls  out  precipitation  will  commence.  The  pre- 
cipitate will  contain,  however,  not  only  the  protein  of  which  the  salt 
concentration  limit  has  been  reached,  but  also  some  of  the  less 
rapidly  precipitable  protein.  The  only  method  available,  therefore, 
for  separating  water-soluble  proteins  from  one  another  is  an  im- 
perfect one  and  this  fact  accounts  for' a  great  deal  of  uncertainty 
as  to  the  homogeneity  of  a  large  number  of  proteins  which  have 
been  submitted  to  investigation."82  Proteins,  by  virtue  of  their  col- 
loidal properties,  can  "  protect "  other  colloids'and  prevent  the  latter 
from  being  precipitated  from  solution. 
At  present  there  is  a  tendency  to  believe  that  the  "  proteins  frac- 
tions of  plasma,  such  as,  globulins,  etc.,  are  not  chemical  individuals, 
but  form  a  series  of  which  the  individual  members  can  pass  one 
into  another.  The  more  complex  proteins  are  assumed  to  be  liber- 
ated from  the  cells  in  a  state  of  low  aggregation  with  compara- 
tively small  amounts  of  absorbed  simple  substances  to  keep  them  in 
a  colloidally  dispersed  state,  from  which  they  can  be  readily  floc- 
culated by  salts,  etc.  In  the  course  of  time,  the  state  of  aggregation 
is  finer  and  the  amount  of  absorbed  substances  is  increased  and  the 
proteins  are  not  then  so  readily  precipitated.  By  alteration  in  the 
state  of  aggregation  and  amount  of  absorbed  simpler  substances, 
variations  in  the  physical  properties  can  be  produced,  and  a  blood 
protein  can  thus  be  varied  in  its  properties  so  as  to  pass  from  a 
readily  aggregating  substance  like  fibrinogen  through  a  globulin  to 
an  albumose."83 
Diphtheria  antitoxin  is  associated  in  some  way  with  that  division 
of  the  proteins  which  is  known  as  the  globulins,  and  is  precipitated 
from  serum  by  agents  which  precipitate  globulins.84  Like  globulins, 
some  antitoxins  are  precipitated  by  heavy  metals.85  However,  Mel- 
lanby  pointed  out  that  the  antitoxin  did  not  correspond  to  the  rigid 
definition  of  a  globulin  (p.  400).  The  globulins  are  insoluble  in 
dilute  acids,  but  are  soluble  in  dilute  alkali  or  in  dilute  neutral  salt 
solution. 
82  Schryver. 
83  Herzfeld,  E.,  and  Klinger,  R.,  abst.  in  Journ.  Chem.  Soc,  Vols.  113- 
114,  p.  87,  pt.  1,  1918. 
84  Brodie ;  Atkinson,  J.  P.,  Journ.  Exper.  Med.,  Vol.  5,  p.  67,  1900-01 ; 
Belfanti,  S.,  and  Carbone,  T.,  Arch.  Sci.  Med.,  1898,  22,  p.  16. 
85  Ditthorn  and  Schultz,  Zeits.  f.  Immunitdts.,  pt.  1,  Vol.  14,  p.  103,  1912. 
