196 G. G. Heslop: 



sitised red blood cells. The M.H.D. of complement is thus 0.25 

 c.c. of a 1 in- 10 dilution. If now we set up a test as follows: — 



l in iu. x - n ia i in 1U. 



Sensitised 

 R.B.C.5%. 



c.c. c.c. c.c. 



16 -•■•16 - '25 - Q.S. to make up to 2.5 c.c. - 1 unit 



and assume that the bovine serum being tested is a positive 

 serum, we would find that complement would be fixed by the: 

 combination of antigen — positive serum; but — and this is the im- 

 portant point — apparently not all the complement is fixed. A 

 certain amount of complement remains unfixed, because it ap- 

 pears that the combination of antigen and antibody in contagious, 

 pleuro-pneumonia is only capable of fixing a small amount of 

 complement. The small amount of complement remaining un- 

 fixed is less than the M.H.D. required to produce haemolysis of 

 the haemolytic system, but it is reinforced by the conglutinin pre- 

 sent in the bovine serum, and thus reinforced, it conglutinates 

 and ultimately haemolyses more or less completely the unit of" 

 sensitised red blood cells added to the test as an indicator. 



In order to overcome errors due to this action of conglutinin- 

 upon a fraction of the M.H.D. of complement, the final reading 

 of the complement fixation test in contagious pleuro-pneumonia. 

 has to be made in strict conjunction with an adequate number 

 of control tubes. These will be fully considered later in the sec- 

 tion, dealing with " Technique." 



In this discussion upon the action of conglutinin, there is an- 

 other point of some importance, which must be referred to. 

 briefly. 



It is probable that conglutinin can bring about a conglutination- 

 reaction, in the presence of complement, with other antigen-anti- 

 body combinations besides the antigen-antibody combination con- 

 tained in the haemolytic system. For instance, the conglutinin in 

 the bovine serum may vary the reaction of our antigen and anti- 

 body (where a positive serum is being used, and especially where 

 culture is used as antigen). If such is the case, the joining up> 

 of the antigen-antibody-complement combination in the first por- 

 tion of the final test for complement fixation may be expected 

 to be delayed considerably, but thereafter sufficient complement 

 may remain free on the addition of the sensitised red blood cells 

 to bring about haemolysis. If this is so, the frequency with which 

 negative complement fixation reactions were obtained in our 



