168 THE VARIOUS TYPES OF IMMUNITY 



immune to the gonococcus, whereas the thin and immature infantile 

 membrane is peculiarly susceptible. 



In general, our knowledge of local immunity is quite incomplete. 

 The subject is a difficult one, hence most attention has been given to the 

 study of general immunity. A striking example of acquired local im- 

 munity may be seen in a patch of psoriasis, where the center is observed 

 to be largely free from scales, whereas the margins are quite active. 



The question of local immunity may be largely determined by vari- 

 ous local non-specific factors, such as loss of blood supply due to trau- 

 matism, thrombosis, tight bandaging, etc., and the action of severe 

 irritants, tending to produce necrosis of the tissues. 



4. The importance of phagocytosis in natural immunity must be em- 

 phasized. Microorganisms are constantly gaining entrance to the 

 tissues through numerous small abrasions of the skin and along the in- 

 testinal and respiratory tracts, and investigations have shown how im- 

 portant the wandering cells are in preventing infection, being ever on 

 guard and ready to pick up and dispose of any injurious material. Even 

 after mild infection has occurred, the local inflammatory reaction in 

 which the phagocyte is a prominent factor may be so prompt in over- 

 coming the invaders that the host will escape serious infection. 



The natural immunity of the frog to anthrax has been shown to be 

 partly dependent upon the activity of the leukocytes in engulfing and 

 disposing the bacilli. 



Similarly a mild irritant may produce hyperemia and exudation or 

 local accumulation of leukocytes, which aid in establishing a local im- 

 munity largely dependent upon phagocytosis. In this manner the 

 intraperitoneal injection of sterile bouillon or even of salt solution may 

 produce exudation and increase the immunity to infection. 



5. It may be that even after the introduction of a microorganism or its 

 toxin no harm results because of a lack of suitable receptors on the part 

 of the body-cells of the host for union with the pathogenic agent. For 

 example, tetanus toxin, being unbound by the cells, produces no 

 effect on the turtle, and antitoxin is not produced. On the other hand, 

 suitable receptors may be present that will bind the toxin, but produce 

 no harmful effects because the body-cells are not susceptible to the 

 action of the microparasite or its products. Thus it is asserted that 

 tetanus toxin has no effect upon the alligator, although the toxin is 

 bound and antitoxin is produced by its body-cells. 



In other instances, a host may escape infection owing to the fact 

 that there is a lowered affinity between a pathogenic agent and the 



