ll>lo] on Anti-Typhoid Inoculation 427 



a. Protective Agents Normally Present. 



1. T\iQ fixed tissue-cells of the lymphatic giands and bone-marrow 

 (indicated by the number 5 in diagram a). 



These, when stimulated by the poisons thrown into the system 

 by the invading bacteria, manufacture the protective sul)stances 

 which play a leading part in the processes of immunization. 



They may be regarded therefore as the arsenals of the invaded 

 state. 



2. The phagocytes, or wandering blood-cells, or — to give them 

 their full title — the polymorphonuclear leucocytes (1 in diagram a). 



These cells have the duty of devouring dead or damaged bacteria ; 

 they thus serve the function of removing the bodies from the battle- 

 field, and may be considered as stretcher-bearers, or — in the friendly 

 parlance of the army — the " body-snatchers." 



It is noted that the substance which actually kills the invading 

 bacteria is contained in the blood-serum, and that its lethal power, 

 as compared with that in normal serum, is known as the " opsonic 

 index." The opsonic index is determined by observing the average 

 number of organisms ingested by phagocytes which have lain for 

 a period in a bacterial emulsion to which blood-serum has been 

 added. 



To the phenomenon of phagocytosis is closely related that of 

 chemiotaxis. Phagocytes are repelled by living organisms, but 

 attracted by dead bacteria, these phenomena being known respec- 

 tively as negative and positive chemiotaxis. 



3. The complement is a substance which is only capable of 

 attacking the invaders if it be provided with the appropriate weapon 

 manufactured by the fixed tissue-cells (4 in diagram a). 



It therefore plays the part of an unarmed defender. 



It is normally present in blood and is destroyed by heat. 



I). Protective Substances which only appear after Invasion. 



1. Antigen, as its name indicates, is the initiator of the produc- 

 tion of anti-bodies (number 2 in diagram a). 



Any extraneous animal substance introduced into the body 

 possesses this power. Unorganized proteins are concerned with the 

 fascinating reactions of anaphylaxis ; organized bacteria concern us 

 to-night as the antigen which, when injected into the human body, 

 leads to active immunization by stimulating the tissue-cells to manu- 

 facture protective anti-bodies. 



The bacteria therefore represent the invading hosts. 



It is as well at this point to answer the frequently asked question 

 — how can the various substances concerned in immunization be 

 accurately compared and have their power measured when they 

 cannot be isolated ? 



