100 THE INDIVIDUAL ORGANISM 



Now let us consider the various stages and events in the history of an 

 infection— an invasion by microorganisms. We have already noted the 

 importance of the skin as a protective layer. Not only does the unbroken 

 surface of the skin interpose a mechanical barrier to entry, but its secre- 

 tions have a bactericidal effect. Let us say that a group of microorganisms 

 enters the skin tissues through an abrasion. If the body has no natural 

 immunity to them, they may rapidly gain access to the circulatory system 

 and be carried through the body, to multiply undisturbed; this is what 

 happens with the syphilis organism. Ordinarily, however, if they are at 

 all common, the body possesses some immunity to them from previous 

 contacts. In this event the first of the protective mechanisms comes into 

 play — localization of the infection. The invading organisms are held 

 within the area of attack until reinforcements are brought by the circula- 

 tion in sufficient quantity to destroy them. All of the tissues of the body 

 have this power of localization to some extent, but the skin is pre-eminent 

 in this ability, as would be expected. 



Immunization enormously increases the capacity of tissues to localize antigens 

 or antigen-containing microorganisms, as Kahn has shown by an ingenious 

 experiment. Diphtheria toxin injected into the blood stream of a rabbit soon 

 kills the animal, but death can be prevented by a simultaneous injection of suffi- 

 cient antitoxin in horse serum. However, rabbits which have been immunized to 

 horse serum localize it in the tissue into which it is injected, and since this inci- 

 dentally prevents the diphtheria antitoxin from reaching the blood these rabbits 

 die. The amount of horse serum antitoxin which they can thus localize is a func- 

 tion of the degree of their immunization. Nonimmunized rabbits could localize 

 in the skin only 15 to 20 units of horse serum antitoxin. By contrast, rabbits which 

 had received two previous injections of horse serum could localize 1,000 to 1,500 

 units, while four or five previous injections would give a localizing capacity of 

 3,500 units. Tissues other than skin had smaller localizing capacities, which were 

 similarly increased by immunization. 



Just how tissues are able thus to block the spread of antigens and of antigen- 

 containing microorganisms is not known, but there is reason to believe that 

 immunized cells unite with the antigens by some sort of colloid chemical bond 

 and thus hold them fixed. If a person with natural immunity to streptococci 

 develops a streptococcic sore throat, it probably means that the surface cells of 

 the throat, upon contact with the microorganisms, have bound them chemically 

 and thus kept them from entering the blood stream. These cells undergo injury 

 and perhaps death to protect the body as a whole. 



The moment tissue injury occurs in a given area of the body, a chain 

 of events takes place aimed at destroying the substance responsible for 

 the injury and healing the injured tissue. Inflammation occurs. This 

 includes the dilation of the capillaries in the area to increase blood supply, 

 the accumulation of fluids and phagocyte cells from the blood, and the 



