430 BACTERIOLOGICAL CHEMISTRY 



more readily than do the others ; these complexes com- 

 bine with antigens, as shown by complement fixation, 

 but do not flocculate, and thus antibody appears to have 

 been destroyed by the heating. Antisera to rod shaped 

 viruses, for example tobacco -mosaic virus or potato -X 

 virus, behave like the flagellar, H -antibodies, whilst 

 antisera to albumin and other more " globular " antigens 

 behave like those to somatic, -antigens. Some anti- 

 bodies are destroyed at the earliest stages of denatura- 

 tion, whilst others may withstand complete denaturation 

 if coagulation is prevented by addition of urea or by 

 dilution with physiological saline. 



The deleterious effect of alcohol on antibodies also 

 runs parallel with the denaturation of the proteins. Below 

 the critical temperature of 4° C. there is no denaturation 

 and no destruction, nor is there if the alcohol concentration 

 is rapidly brought above 90 per cent. The addition of 

 acid or alkali accelerates the effect of alcohol. 



The Effect of Chemical Changes on Antibodies. — 



Antibody globulins, like normal globulins, are only slowly 

 attacked by trypsin but are much more readily destroyed 

 by pepsin. Brief treatment with pepsin at pK 4 and 37° C. 

 causes the antibody globulin of diphtheria antitoxin, for 

 example, to break into two parts, one of which does not 

 carry antitoxic activity and is easily denatured and 

 coagulated by heating at 60° C. for fifteen minutes, 

 whilst the active part is much more resistant to heat and 

 remains in solution. By precipitation of the active 

 fraction by ammonium sulphate after removal of the 

 easily coagulated fraction by heating, a considerable 

 concentration and purification of the antibody can be 

 achieved. During the digestion by pepsin the T com- 

 ponent (see p. 426) disappears and is replaced by the 

 slower moving y-globulin. More prolonged digestion 

 causes further breakdown into fragments which can no 

 longer flocculate with antigen but can still react with it 



