IMMUNITY AND SUSCEPTIBILITY 693 



It is evident, therefore, that the protection afforded an individual 

 by the body surfaces is a decided factor in the natural immunity of 

 that individual. 



The Protective Nature 0/ Inflammatory Processes. — It has been 

 mentioned in a previous di-scussion that when bacteria successfully 

 enter a tissue and develop in that tissue a complex local change results 

 which is designated as inflammation. In the majority of instances 

 inflammation is of a beneficial nature. Fundamentally, it is always 

 beneficial. Few examples of the pernicious results from inflammation 

 can be given. In this connection may be mentioned the thickening 

 of the cerebral blood-vessels in syphiUs and the increase of connective 

 tissue in cirrhosis of the liver. In these instances the inflammatory 

 processes are brought about by the reaction of the various tissues to 

 the irritation of the infecting microorganisms. Unluckily these reac- 

 tions are not on the whole beneficial to the body, but, as before stated, 

 inflammation is usually beneficial and may be characterized as the re- 

 action of tissues to injury. The exact processes of inflammation may 

 be traced in case an infecting microorganism succeeds in entering the 

 tissues of the body. The organism having produced its toxic substance 

 first causes a congestion of the blood-vessels in the region (hyper- 

 emia). Following this localized congestion there is an extravasation 

 of plasma from the blood-vessels. This plasma immediately on leav- 

 ing the vessels coagulates or clots, producing throughout the infected 

 area fibrin and blood serum. This fibrin serves in a mechanical way 

 to limit the infection, and it has been recently demonstrated that 

 the fibrin possesses germicidal properties in addition. Furthermore, 

 the serum in a large number of instances exerts a bactericidal effect 

 upon the microorganisms. Following the extravasation of blood 

 plasma from the capillaries, the leucocytes pass out and gather around 

 the infected area. These leucocytes are attracted to the area due 

 to the presence of various chemical substances (chemotaxis). They 

 will come as close to the microorganisms as possible, depending upon 

 the effect of the toxins which have been produced. In certain in- 

 stances they will ingest the bacteria and destroy them. In such 

 cases, the bacteria having been removed, the inflammation rapidly 

 subsides and the infection is, therefore, checked. Such are the char- 

 acteristics of an acute inflammation. Inflammations, however not 

 infrequently depending especially upon the microorganism producing 



