138 NATURAL IMMUNITY TO TOXINS 



has brought forward very strong (though not conclusive) evidence 

 in support of his theory of the leucocytic origin of antitoxin ; and, 

 as we shall see, as far as our knowledge goes, we are led to believe 

 that all antibodies are derived from the spleen, bone-marrow, and 

 other organs rich in leucocytes. 



The study of the affinity of bacterial toxins and of other poisons 

 to various tissues and organs of the body is destined to play a part 

 of great importance in our ideas of pathology and pharmacology. 

 It has been especially investigated by Ehrlich, and in connection 

 with immunity and susceptibility to toxins he recognizes four 

 conditions which may occur : 



1. In which no specific receptors occur in any part of the 

 animal. This is the first form of natural immunity which we 

 have discussed ; the animal is absolutely immune, and cannot 

 produce antitoxin. 



2. Receptors are present, but only in tissues on which the 

 poison does not act, or in tissues of little importance. Here the 

 animal is immune, but antibodies may be produced. This is the 

 case of the fowl vis-a-vis the tetanus toxin. 



3. The receptors are distributed over various parts of the 

 organism, and are present in the organs which are sensitive to the 

 action of the poison. Here there is a relative immunity, and the 

 degree of intoxication depends largely on the region into which 

 the toxin is introduced. These animals may be immunized, 

 though the process is one of some difficulty, owing to the sensitive- 

 ness to the toxin of vital organs, and, of course, antibodies may 

 be produced. 



4. The receptors are present only in vital organs which are 

 sensitive to the poison. Here the animal is very susceptible, and 

 immunization very difficult, involving the use of extremely minute 

 doses or of toxoids. The action of tetanus toxin on the guinea- 

 pig and horse provides examples. 



