IMMUNITY AND IMMUNIZING AGENTS. 1 17 



for injecting. An animal inoculated with the typhoid bacillus will produce 

 a blood lysin which destroys the typhoid bacillus. Lytic sera become in- 

 active when heated to 55 C. for one-half hour and such sera are said to be 

 inactivated. However, if normal serum is added to the inactivated serum 

 the bactericidal power is fully restored. The bactericidal power of the 

 serum can be greatly increased by the use of highly virulent bacterial cultures, 

 thus producing a serum of high potency. In actual practice, as in the man- 

 ufacture of bactericidal sera for the prevention and cure of disease, the ani- 

 mal (as horse) is first inoculated with attenuated cultures, then with normally 

 virulent cultures and finally with hyper-virulent cultures of the specific 

 pathogenic microbe. Such sera act by destroying the disease-producing 

 bacteria, but they have no effect upon the toxins produced by the bacteria, 

 thus showing that they are entirely distinct from the antitoxins. 



The eminent bacteriologist Metchnikoff made the very interesting 

 discovery that the white blood-corpuscles (leucocytes) had the power of 

 feeding upon and digesting bacteria with which they came in contact. That 

 is the white blood-corpuscles, called phagocytes, act as the defenders of the 

 body against bacterial invasion. This observation by Metchnikoff, fully 

 verified by others, is generally known as the phagocyte theory and the 

 phenomenon is designated phagocytosis. The principle involved in phago- 

 cytic activity is well illustrated in the lesser local injuries, as cuts, bruises, 

 abrasions, etc. Normally such injuries are always infected by various 

 germs of the environment, as the several varieties of pus microbes. These 

 invading microbes at once begin their attack upon the tissue cells and 

 blood-corpuscles. The leucocytes which are present begin to feed upon 

 the rapidly multiplying pus organisms but for a time, as a rule, the latter 

 have the upper hand and as a result there is perceptible pus formation 

 ("the laudable pus" of older writers) represented by dead leucocytes gorged 

 with microbes. As the inflammatory reaction becomes more marked, in- 

 dicated by redness and swelling of the tissues immediately about the injury; 

 increased numbers of leucocytes (phagocytes) are brought to the scene of 

 action and gradually they gain control until finally the invading microbes are 

 all destroyed, thus permitting a rapid and unhindered restoring of tissue cells, 

 recognized as the healing process. This phagocytic action is entirely 

 distinct from the action of antitoxins and lysins, and the three are potent 

 factors in immunity. 



The investigations of Metchnikoff and Leishman on phagocytosis paved 

 the way for the discovery of opsonins by Wright. It was noticed that the 

 phagocytic activity was influenced by conditions to be found outside of the 

 leucocytes themselves. Metchnikoff held that the principal part is played 

 by substances found in the serum and in the tissue cells to which he gave the 

 name "stimulins." The purpose of these substances in the tissue fluids 



