588 IMMUNITY 



a large amount of antitoxin, how does the additional toxin 

 injected reach the cells in order to influence them as we know 

 it does ? This also is difficult to understand, unless the toxin 

 has a greater affinity for the receptors in the cells than for 

 the free receptors (antitoxin) in the serum. A supersensitive- 

 ness of the nerve-cells of an animal to tetanus toxin, sometimes 

 observed even when there is a large amount of antitoxin in 

 the serum, has been often brought forward as an objection. 

 But this also may perhaps be explained by there having occurred 

 a partial damage of the cell protoplasm by the toxic action in 

 the process of immunisation — an explanation which, of course, 

 demands that in some way the freshly introduced toxin may 

 reach the cells in spite of the antitoxin in the blood, or it may 

 belong to the group of anaphylactic phenomena described below 

 (p. 595). Further investigation alone will settle these and 

 various other disputed points, and may remove many of the 

 apparent objections. At present we may say, however, that 

 Ehrlich's theory is the only one which even attempts to explain 

 the cardinal facts of this aspect of immunity. 



2. The Theory of Phagocytosis. — This theory, brought 

 forward by Metchnikoff to explain the facts of natural and 

 acquired immunity, has been of enormous influence in stimu- 

 lating research on the subject. Looking at the subject from the 

 standpoint of the comparative anatomist, he saw that it was 

 a very general property possessed by certain cells throughout 

 the animal kingdom, that they should take up foreign bodies 

 into their interior and in many cases digest and destroy them. 

 On extending his observations to what occurred in disease, he 

 came to the conclusion that the successful resistance of an 

 animal against bacteria depended on the activity of certain cells 

 called phagocytes. In the human subject he distinguished two 

 chief varieties, namely — (a) the microphages, which 'are the 

 " polymorpho-nuclear " finely granular leucocytes of the blood ; 

 and (6) the macrophages, which include the larger hyaline 

 leucocytes, endothelial cells, connective tissue corpuscles, and, in 

 short, any of the larger cells which have the power of ingesting 

 bacteria. Insusceptibility to a given disease is indicated by a 

 rapid activity on the part of the phagocytes, different varieties 

 being concerned in different cases, — an activity which may 

 rapidly destroy the bacteria and prevent even local damage. If 

 the organisms are introduced into the tissues of a moderately 

 susceptible animal, there occurs an inflammatory reaction with 

 local leucocytosis, which results in the intracellular destruction 

 of the invading organisms. Phagocytosis was regarded by 



