110 A TEXTBOOK OF PHYSIOLOGY 



The Complement is believed to be the actual destroying agent. It is not increased 

 in the blood during the process of immunization, and thus it comes about in some 

 cases that there is not sufficient complement for all the immune substances whose 

 action it is sought to demonstrate. A sufficiency can be provided by the addition 

 of normal serum. The origin of the complement is not known. The leucocytes 

 and the tissues may each play a part in providing it. The complement is easily 

 destroyed by heating to 56 C. While its chemical nature is quite unknown, it is worth 

 noting that in the case of snake venom the phosphorized fat, lecithin, plays the part 

 of, or is associated with, the complement. 



The Deviation of the Complement. When serum containing a specific immune 

 body is inactivated (has its complement destroyed) by heat and mixed with the 

 antigen used in its production, a combination takes place between the two, and the 

 antigen is then said to be sensitized. However, no visible effect is apparent until 

 complement contained in fresh serum, usually that of a guinea-pig, is added. The 

 complement combines with the sensitized antigen, and may produce a visible effect 

 (e.g., haemolysis). It should be noted that an antigen can only be sensitized by its 

 specific immune body, and that complement can only combine with the antigen when 

 linked to the specific immune body; thus, the combination, or fixation, of the com- 

 plement can be made a test for the presence of a specific immune body. The test 

 is very delicate and of great diagnostic value, and is carried out in the following way: 



A rabbit is immunized against sheep's blood-corpuscles. Its serum is obtained, 

 heated to 56 C., and kept in sealed capsules. This serum will haemolyze sheep's 

 corpuscles if a certain minimal amount of normal serum of a guinea-pig is added. 

 The minimal amount is determined by experiment. All is now ready for testing the 

 blood, say, of a man suspected to be infected with typhoid bacilli. Serum is obtained 

 from this man and heated to 56 C. to destroy the complement in it. It is then mixed 

 with typhoid bacilli and the minimal amount of normal guinea-pig serum added. 

 The mixture is kept at body temperature, and time enough allowed for the specific 

 immune body (if present) to fix the complement and antigen (the typhoid bacilli). 

 It is then added to a mixture of sheep's corpuscles and the heated rabbit's serum. 

 Haemolysis will not take place if the complement has been fixed in the first stage of the 

 test, since none will be left to combine with the sensitized sheep's corpuscles; and in 

 such a case it is clear that the suspected serum did in fact contain typhoid immune 

 bodies. If haemolysis does take place, the complement could not have been fixed 

 in the first stage, and thus the suspected serum was not from a case of typhoid. 



Opsonins. These are specific substances in the serum which act on bacteria in 

 such a way as to make the phagocytes ingest them. Their presence is demonstrated 

 thus : Blood is collected and allowed to clot. The white corpuscles are separated from 

 the serum by means of the centrifuge. The serum is pipetted off, and the corpuscles 

 mixed with physiological salt solution, and again separated by the centrifuge. This 

 procedure washes the corpuscles free from serum. Bacteria are mixed with the 

 washed corpuscles and the mixture kept at body temperature for ten minutes. A 

 film is then made, stained, and the average number of bacteria ingested by the phago- 

 cytes counted. A similar experiment is performed, only in this case the serum is 

 allowed to act on the bacteria. The phagocytes ingest many bacteria which have been 

 first acted on by the serum, and very few of those which have not been so treated. 

 Thus, the serum contains opsonin which prepares the dish for the leucocytes to ingest. 

 Opsonins exist in the normal blood of many animals, and are increased in amount 

 by the process of immunization by vaccination with dead bacteria. The opsonins 

 in the serum of one animal are able to act on the bacteria, so that they are ingested by 

 the phagocytes taken from another animal. Their chemical nature is not known. 

 They are of the utmost importance in furthering the defence of the body by the 

 phagocytes. 



Agglutinins. These are bodies possessing the property of clumping bacteria. 

 The bacteria themselves are not greatly damaged by the process, but it tends to pre- 

 vent the dissemination of the organisms, and it may in some way favour phagocytosis. 

 The nature of the change thus brought about in the bacteria is not well known, but 

 their colloidal nature is probably altered by changes in surface tension. Agglutinins 

 can be prepared against almost all bacteria. Their place of formation is not known. 

 They have been demonstrated in the blood and to a less extent in the milk. Attachc'l 

 in some way to the globulin in the plasma, they cannot be separated from it. They 

 are destroyed by heat; the temperature of destruction varies for different agglutinins. 

 As in all processes of a like nature the concentration of the electrolytes in solution 

 affects their action. 



