18 



VACCINE AND SERUM THERAPY. 



RECEPTORS OF THE THIRD ORDER 



The receptors of the third order are likewise adapted to the 

 anchorage of bacteria. These receptors, however, have two com- 

 bining groups: one for the anchorage of cells or food substances 

 and the other for the anchorage of substances which, acting through 

 the receptors, can produce a ferment-like action. This latter 

 substance is called an activating substance and is present in nor- 

 mal sera. It is known either as complement or alexin. 



Complement is easily destroyed by ageing, acids, heat, 55 C., 

 being sufficient to destroy it. When the receptors of this order 

 are anchored by bacteria in numbers not sufficient to kill the cell 

 but to stimulate it to over-production of receptors, the extra 

 receptors are thrown off. The receptors thrown off constitute 

 the immune body, amboceptor or substance sensibilitrice of Bor- 

 det. They have two combining or haptophore groups, one for 

 the combination with bacterial or other cells and substances, 

 called the "cytophylic" group; the other for combination with 

 alexin or complement, called the "complementophore" group. 

 It is by means of complement that the amboceptor is able to 

 dissolve bacteria, red blood cells and other substances. Immune 

 sera containing receptors of the third order are bacteri- 

 olytic, haemolytic, or cytolytic, depending upon whether they, 

 together with complement, can dissolve bacteria, red blood cells, 

 or other cells. The receptors of the third order are graphically 

 represented in Figure 3. 



FIG. 3. 



It will be observed that receptors of the first, second or third 

 order can be produced in excess to form immune body or anti- 



