414 THE QUESTION OF IMMUNITY AND ANTITOXINS 



certain cells in the tissues the powers of " scavenging," overtaking 

 germs of disease, and absorbing them into their own protoplasm. 

 This, indeed, may be actually witnessed, and had been observed before 

 the time of Metchnikoff. But he it was who applied the observation 

 to the destruction of pathogenic organisms. He came to the con- 

 clusion that the successful resistance which an animal offered to 

 bacteria depended upon the activity of these scavenging cells, or 

 phagocytes. These cells are derived from various cellular elements 

 normally present in the body : leucocytes, endothelial cells, connective 

 tissue corpuscles, and any and all cells in the body which possess 

 the power of ingesting bacteria. If they were present in large 

 numbers and active, it was argued, the animal was insusceptible to 

 certain diseases; if they were few and inactive, the animal was 

 susceptible. It appears that the bacteria or other foreign bodies in 

 the blood which are attacked by the phagocyte become assimilated 

 until they are a part of the phagocyte itself. Metchnikoff explained 

 how the phagocyte is able to encounter bacteria when both are 

 •circulating through the blood. It is guided, he holds, in this attack 

 on the organisms by the power of chemiotaxis. The bacteria elaborate 

 a chemical substance which attracts the phagocyte, and this is 

 termed " positive chemiotaxis." But it may occur that the chemical 

 substance produced by the bacteria may have an opposite, or repellent, 

 effect upon the leucocytes, in which case we have " negative chemio- 

 taxis." Metchnikoff distinguishes two chief varieties of phagocytes 

 which become active in disease : (a) the microphages, which are the 

 polynuclear leucocytes of the blood, and (&) the macrophages, which 

 include the larger hyaline leucocytes, connective tissue cells, etc. It 

 is now known that blood serum, from which all leucocytes (phagocytes) 

 have been removed, possesses immunising effects as before, it is 

 therefore clear that such effect is a property of the serum per se, and 

 not wholly or only due to the scavenging power of certain cells in it. 

 Metchnikoff explains this fact by stating that the phagocytes possess 

 digestive ferments (cytases) which may be set free in the blood 

 serum, giving it its bactericidal properties. Metchnikoff admits that 

 antitoxin and toxin form a neutral compound, but holds also that 

 acquired resistance of body cells is of importance in toxin immunity. 

 5. Ehrlich's Side Chain Theory. — Ehrlich looks upon a molecule 

 of protoplasm as composed of a central atom cell with a large 

 number of side chains of atom groups. The central cell is the 

 mother cell, the side chains are receptors, that is, cells having com- 

 bining affinity with food stuffs by which nutriment is brought to 

 the mother cell. These receptors are of two kinds, those having 

 power of combining with molecules of simple constitution, and those 

 having power of breaking up compound bodies by ferment action for 

 the purposes of assimilation. Now if toxins be introduced into the 



