54 BACTERIOLOGY. 



the development of the same species of micro-organisms hereafter ; 

 this condition may be -permanent, or the chemical constitution of 

 the tissues may be restored to normal, when immunity ceases. If, 

 however, we explain acquired immunity by the result of the growth 

 of a previous invasion of micro-organisms, we are still confronted 

 with the difficulty of explaining natural immunity. 



A third theory is that the tissues are endowed with some 

 power of vital resistance to the development of micro-organisms, 

 similar to the vital resistance to coagulation of the blood, which is 

 supposed to exist in the lining membrane of the healthy blood- 

 vessel; that in some species arid individuals this exists to a high 

 degree, and hence their natural immunity. But this does not 

 explain how one attack confers immunity from a subsequent one. 

 One would expect that the vital resistance would invariably be 

 lowered by a previous attack, and increased liability be the constant 

 result. 



A fourth theory was propounded by Metchnikoff, who maintains 

 that immunity depends upon phagocytosis. If anthrax bacilli are 

 inoculated in the frog, white blood-cells, or phagocytes, are observed 

 to incorporate and destroy them until they entirely disappear, and 

 the animal is not affected. But if the animal, after inoculation, 

 is kept at a high temperature, the bacilli increase so rapidly that 

 they gain the upper hand over the phagocytes, and the animal 

 succumbs. 



It has also been suggested that bacteria may attract or repel 

 the phagocytes, exercising either a positive or a negative chemio- 

 taxis. This power is supposed to depend upon some special product 

 of the bacteria or possibly upon their toxins, as suggested by Roux. 

 We must suppose that the negative chemio-taxis has become changed 

 to a positive chemio-taxis in an immunised animal, so that the 

 phagocytes, instead of withdrawing and leaving the bacteria to 

 multiply, are readily drawn into the contest and destroy the 

 invaders. 



In septicaemia of mice, the white blood-cells are attacked and 

 disintegrated by the bacilli in a remarkable way. It is difficult, 

 however, to accept these observations as affording a complete ex- 

 planation of immunity. It is difficult to conceive that the leucocytes 

 in the blood and tissues in the field mouse are differently constituted 

 from those in the house mouse, so that they form an effectual 

 barrier to the onset of bacteria in the one case, though so readily 

 destroyed in the other, or that in acquired immunity the result is 

 due to educating the phagocytes to respond to a positive chemio-taxis. 



