president's address — SECTION L.. 279 



In an animal immunized against i"ed blood corpuscles we have 

 seen that the blood serum beccmes htemolytic to the particular kind 

 of ccipu&cles used. Placed in a test tube, these corpuscles will be 

 speedily dissolved en the addition of a small amount of the immune 

 serum diluted with saline solutiori. Bvit if the immune serum be 

 heated to 56" C. for half -an -hour it becomes ineffective, and yet 

 once again it can be reactivated by the addition of a little fresh 

 normal serum from (practically) any animal. OlDviously the 

 immune serum must be made up of at least two factors, one of 

 which is destroyed or inactivated by heating to 56°, whilst the 

 other remains intact. Further, the thermolabile substance is evi- 

 dently non-specific, for it can be supplied by any fresh serum. 



This thermolabile constituent of serum, whether normal or 

 immune, is Cumplenicnt. 



But not only is it a factor in haemolysis, but in bacteriolysis and 

 )nany other immunity i eactions ; in other words, the phenomenon 

 of lysis is the result of a combination of three factors, the anfif/oi, 

 whether red cells, bacteria, or ethers; the anfibod//, a specific sub- 

 stance produced in thei animal as a result of infection or vaccina- 

 tion ; and com idem cut , a normal constituent of the blood serum of 

 animals. Tins constituent varies in amount in different animals, 

 but is present in high concentration in the serum of the guinea pig, 

 and for laboratory purposes this animal is the usual source of 

 complement. 



To determine whether the blood serum of a particular animal or 

 man contains immune bodies against a particular disease (as would 

 be the case if the animal were norvv, or had been, infected with that 

 disease) it would suffice to show that the serum could unite with 

 a small quantity of the bacteria, so forming the complex antigen- 

 antibody-complement. If, then, the sample of serum can bedispos- 

 sessed of its own unknown quota of complement, and if a known 

 minimal quantity of complement can be added to the mixture of 

 bacteria and suspected serum, any reaction between them will 

 entail the fixation of this ]ninimal quantity of complement, for it 

 is essential to the reaction. Unfortunately, the reaction gives no 

 obvious indication of having taken place, but we may now deter- 

 mine whether the couiplement has been fixed or is still free by 

 adding to the mixture some sensitized red corpuscles. Thev have 

 already been siibmitted to the action of a htemolytic serum, devoid, 

 however, of complement. All that is necessary for their haemolysis 

 is the addition of a minimal quantity of free complement. So, if 

 our first mixture, bacteria, immune serum, and one unit of com- 

 ]}lement, has combined to fonn a ccmplex, the red cells subse- 

 quently added will remain intact, for the complement is fixed in 

 the first combination. Conversely, if the bacteria found no immune 

 body in the suspected serum, no fixation of complement would 

 occur, and this would remain free, and, on the addition of the 

 sensitized red cells, would unite with them and produce hgemolv.'ii« 

 y.t. , liberation of the hsemoglobin. 



