172 PRINCIPLES OF BACTERIOLOGY 



dant in the spleen and the hsemopoietic organs and also to appear there 

 first during the process of immunization. 



The Part Played by the Leukocyte in Immunity . The original theory of 

 Metchnikoff, that the leukocytes were the only actual protective bodies 

 which warded off disease, and that they did this by attacking the bacteria 

 was founded on the now well-known fact that certain of the white cells 

 possessed the power of taking up into themselves pathogenic bacteria, 

 and that they were there destroyed. It was later observed that these 

 cells had the property of taking from the blood many lifeless foreign 

 elements, thereby keeping the blood channels free of foreign particles. 



The question thereby arose as to whether these cells engulfed and 

 then killed the bacteria, or whether perhaps other substances killed or 

 prepared them before the cells took them up. It is now known that 

 the bacteria first unite with substances in the serum and thus are pre- 

 pared for phagocytosis. This union does not kill the bacteria. The 

 leukocytes and the chemical substances of the blood thus both play an 

 important part. The death of the bacteria also liberates positive 

 chemotactic substances, and the disintegration of the white blood cells 

 gives rise to bactericidal bodies. We find that phagocytosis is most 

 marked when the disease is on the decline or the infection mild, but is 

 usually absent in rapidly increasing infection. This would seem to 

 indicate that the course of the infection is often already determined 

 before the leukocytes become massed at the point of its entrance. The 

 first determining influence is given by the condition of the tissues and 

 the amount of bactericidal substances contained in them, and then, 

 later, in cases where the bacteria have been checked, comes the addi- 

 tional help of the leukocytes. If the tissues are wholly free of bacteri- 

 cidal and sensitizing substances, neither they nor the leukocytes, nor 

 both combined, can prevent the bacterial increase. The simple ab- 

 sorption by the cells of bacteria is not necessarily a destructive process. 

 Metchnikoff believes that the polymorphonuclear leukocytes are espe- 

 cially antibacterial in relation to acute infections. The large phagocytes 

 are conceived to deal chiefly with the resorption of tissue cells and with 

 immunity to certain chronic diseases, such as tuberculosis. 



Opsonines 1 or Substances which Prepare the Bacteria for the Leukocytes. 

 Wright, Neufeld, and others have studied the substances in the blood 

 which prepare the bacteria for the leukocytes. The interesting fact has 

 been noted that the white blood corpuscles of the non-immune and the 

 actively immune have the same phagocytic characteristics toward bac- 

 teria, the differences in their apparent activity being due to the sub- 

 stances in the serum which, similar to the immune body, combine with 

 the receptors in the cells. 



If the washed cells from the immune patient are added to salt solu- 

 tion they have no more effect upon the bacteria used to immunize than 

 do those from a susceptible person, while those from the latter put into 

 the serum of the former act energetically. Heating destroys all opsonines 



1 Greek " Opsono "I cater for. 



