REACTION OF THE HOST TO INFECTION 207 



Carl Weigert, serves to explain tissue hyperplasia and repair 

 following exercise or local destruction of tissue. Examples of 

 these phemomena will occur to the reader. 



Phagocytosis and Encapsulation. The mere physical mass of 

 a parasite within the tissue acts as a foreign body and it becomes 

 surrounded by tissue elements. If it is minute, certain cells of 

 the body (phagocytes) flow around and ingest it, as was first 

 observed by Metchnikoff. If it is larger, the connective tissue 

 cells proliferate and surround it, and eventually contract into a 

 firm capsule. Further, the tissues produce enzymes capable 

 of dissolving many foreign substances introduced in this way 

 (parenteral digestion). If the foreign body is insoluble, it will- 

 remain encapsulated, or, if sufficiently minute, it may be trans- 

 ported considerable distances inside wandering cells and eventu- 

 ally be deposited in a lymph gland. The wholly passive para- 

 site or the dead body of a micro-organism is therefore either 

 digested and dissolved, ingested by cells, or encapsulated in 

 fibrous tissue. Most infectious agents are not passive in this 

 way, as we have seen, but tend actively to grow and multiply, 

 to absorb and utilize food material, and, most important of all, 

 to produce various substances which stimulate or poison the 

 cells of the host. Against these the physical measures of inges- 

 tion (phagocytosis) and encapsulation are often inadequate de- 

 fenses and may be entirely useless. 



Chemical Constitution of the Cell. Ehrlich has compared 

 the living body cell to a complex chemical molecule; in fact it 

 may be said that he regards the living cell as an enormous mole- 

 cule, a chemical unit of great complexity. Certain atom groups 

 within this molecule are pictured as relatively very stable and 

 they constitue the chemical nucleus (not to be confused with 

 the anatomic nucleus). Grouped about this chemically stable 

 center are very many, more labile atom groups which readily 

 enter into chemical reaction with substances in the surrounding 

 medium. The conception is derived directly from well-known 

 facts in organic chemistry. For example when benzoic acid, 



