THEORIES OF IMMUNITY 129 
the bacterium disappeared entirely. His attention was soon directed 
to a small, transparent crustacean, daphnin, within whose body cavity 
could be distinguished minute wanderino; cells which exhibited amoeboid 
movements. The physiological significance of these amoeboid cells— 
which are potentially leukocytes— was not clear until it was found 
that they engulfed and digested certain yeast spores that occasionally' 
gained entrance to the body cavity of the crustacean. If the yeast 
spores were not too numerous the wandering cells flowed around 
and eventually destroyed them; if, on the contrary, the number of 
yeast spores was too great, the wandering cells could not remove the 
entire number and the residual spores germinated within these cells, 
and killed the host. It was evident that the phagocytic activity of 
the amoeboid cells played a prominent part in protecting daphnia from 
an infection with the yeast. 
Next Metchnikoff injected anthrax bacilli into the lymphatic sac 
of frogs and found again that wandering cells — leukocytes— engulfed 
and destroyed the bacteria, thus preventing infection and death of 
the frog. This line of observation was followed through an exten- 
sive series of lower animals, mammals, and finally in man, w^here 
the engulfment of the meningococci, gonococci, pneumococci and 
staphylococci by polymorphonuclear leukocytes during the course of 
acute infection with these organisms afforded a striking demonstra- 
tion 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, reinforced 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. 
Metchnikoft' classified phagocytic cells of the body into two groups: 
1. Macrocytes or Macrcyphages.— Large mononuclear cells and certain 
fixed 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 Micropluigc.'i. — Chiel^y polymorphonuclear leuko- 
cytes which occur in the blood stream. They engulf ])acteria and 
similar cells. IMicrocytes 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 
9 
