132 IMMUNITY AND INFECTION 
at 37° C. (a much longer exposure at 0° to 4° C. was necessary), then 
washed free from adherent serum or plasma, and exposed to washed 
leukocytes, active phagocytosis took place. 
McKendrick,! in a thoughtful study of the physical theory of 
phagocytosis, does not afford much support to the opsonic theory. 
He says, "When an assemblage of leukocytes operates on an assem- 
blage of particles, and ingestion occurs, it is well known that at any 
subsequent instant of time, the numbers of leukocytes which contain 
0, 1,2, n, particles are derived by the successive terms of a 
binomial series, the index of which, as far as experience goes, is invari- 
ably a negative quantity." Experiments (not detailed) suggest that 
this numerical value indicates the ratio between phagocytic and 
agglutinative avidities. 
The present tendency is to ascribe to phagocytosis an important 
part both in the destruction of many kinds of invading bacteria and 
in the removal of alien or abnormal cells as well. The importance 
of substances contained within the plasma or blood serum, which 
prepare bacteria for phagocytosis— "opsonins" to use Wright's ter- 
minology—has modified somewhat the original conception of phago- 
cytosis as proposed by Metchnikoff . 
The phagocytic theory and the humoral theory of immunity would 
appear to be in direct opposition. Metchnikoflf maintained that the 
fundamental basis of immunity resides in the phagocytic activity 
of macro- and microphages. It must be remembered, however, that 
the same kind of phagocyte operates against a variety of microbes : the 
polymorphonuclear leukocytes in general against the bacteria that 
cause acute infections, the mononuclear leukocytes in association with 
the chronic types of infecting agents. Also, it is important to remem- 
ber that leukocytes are short lived, and therefore the ^'eterans of one 
encounter do not as a rule survive to serve as a nucleus for another de- 
fensive cellular army. In general, therefore, the Metchnikofl' theory 
appears to be applicable particularly in the defense of the body against 
invasion by "opportunists," which have no definite portal of entry to 
specific tissues, and which do not habitually escape from tissues into 
which they may have accidentally gained entrance. The leukocytes, in 
other words, seem to be a constabulary, patrolling the highways and the 
byways of the body, picking up occasional accidental invaders, but 
unable to cope with those bacteria that invade the tissues at particular 
points, and escape from the tissues at specific places. These latter 
bacteria are gotten rid of by specific antibodies, as a rule. 
Summary.— The phagocytic theory, therefore, is a defensive theory, 
explaining the mode of removal of accidental invasion by parasitic 
organisms. It is not, therefore, a theory explaining immunity to 
specific infection. The humoral theory on the contrary is intended to 
explain the oi)eration of the specific ofl'ensive mechanism which frees the 
body from alien, habitually invading bacteria.^ 
' Jour. Physiol., 1925, 60. XXVII. 
2 See Kendall: Joyr. Infec. Dis., 1923, 32, ,341, for discus.sion. 
