128 IMMUNITY AND INFECTION 



of the phagocytic activity of leukocytes which circulate normally in 

 the blood and lymph streams. These and many other observations 

 and experiments led to the formulation of the phagocytic theory of 

 immunity. Natural immunity, according to this theory, is leukocytic 

 immunity that is, the natural barriers of the body, reenforced by 

 the activity of leukocytes in the blood and lymph streams which 

 bathe the intact skin, mucous membranes, etc., suffice to protect 

 the body against invasion by moderate numbers of bacteria or other 

 microorganisms. Infection of the body, according to this view, is 

 attributable to a failure of the leukocytic defense, or to too large 

 numbers of invading organisms, or both factors combined. 



Metchnikoff classified phagocytic cells of the body into two groups : 



1. .Macrocytes or Macrophages. Large mononuclear cells and certain 

 fixed tissue cells, particularly of the spleen, liver, lungs, and lymph 

 nodes. Macrophages are active in the removal of necrotic tissue, 

 injured blood cells, and similar abnormal cellular elements of the 

 body, and in chronic bacterial' infections, notably in tuberculosis, 

 leprosy, and actinomycosis. They contain a digestive enzyme 

 macrocytase which dissolves or digests these abnormal cells. 



2. Microcytes or Microphages. Chiefly polymorphonuclear leuko- 

 cytes which occur in the blood stream. They engulf bacteria and 

 similar cells. Microcytes contain a digestive enzyme microcytase 

 which dissolves or digests bacteria. 



The substance which Ehrlich regards as complement is normally 

 present in the leukocytes as macro- and microcytase, according to 

 Metchnikoff. These cytases are liberated into the blood stream when 

 the leukocytes are destroyed (phagoly sis) . 



The phenomenon of phagocytosis may be divided into three separate 

 and distinct phases: the method of approach of the phagocytic cell 

 to its prey (chemotaxis), the engulf ment, and finally the digestion or 

 destruction of the latter. 



The Method of Approach. It was a matter of observation by Metch- 

 nikoff and his followers that phagocytosis was more marked in mild 

 bacterial infections and during recovery than in severe infections and 

 the early acute stages of the disease. The importance of chemotaxis 

 as the attractive force of leukocytes to bacteria, however, was not 

 clearly realized until Massart and Bordet 1 showed by ingenious 

 experiments that non- virulent bacteria apparently secrete substances 



1 Ann. Inst. Past., 1891, v, 417. 



