ACTIVE IMMUNISATION 325 



mental work a sufficient amount of any required anti- 

 body can often be obtained without carrying the process 

 of immunisation to its logical termination. 



For instance, if the immunisation of a rabbit toward 

 Bacillus typhosus is commenced on the lines already 

 set out it will often be found, after a few injections of 

 "killed" cultivation that the blood serum of the animal 

 (even when diluted with several hundred times its 

 volume of normal saline) contains specific agglutinin 

 for B. typhosus and if the sole object of the experi- 

 ment has been the preparation of agglutinin the inocu- 

 lations may well be stopped at this point, although the 

 animal is not yet immune in the strict meaning of the 

 word. 



Again, antibodies may be formed in response to 

 antigens other than infective particles thus the injec- 

 tion into suitable animals of foreign proteins such as 

 egg albumin, heterologous blood sera or red blood discs 

 from a different species of animal, will result in the 

 formation of specific antibodies possessing definite 

 affinities for their respective antigens. 



The most important antibody of this latter type is 

 Haemolysin, a substance that makes its appearance in 

 the blood serum of an animal previously injected with 

 washed blood cells from an animal of a different species. 

 The serum from such an animal possesses the power of 

 disintegrating red blood discs of the variety employed 

 as antigen and causing the discharge of their contained 

 haemoglobin, and is specific in its action to the extent 

 of failing to exert any injurious effect upon the red 

 blood cells of any other species of animal. 



The action of this serum is due to the presence of 

 two distinct bodies, complement and haemolysin. 



Complement (or alexine) is a thermo-labile readily 

 oxidised body present in variable but unalterable 

 amount in the normal serum of every animal. It is 

 a substance which exerts a lytic effect upon all foreign 



