Future of Infectious and Malignant Diseases 
like to know why half the readers of this symposium will suffer 
from five to ten common colds during the coming year while the 
the other half will remain free of this affliction, even though the 
frequency of exposures will be the same for the two groups. 
We know only that this disparity in susceptibility cannot be 
explained by the presence of specific antibodies. 
Although no approaches have so far been made to investigate 
this problem rationally in man, data from animals may indicate 
pathways to be followed in the future. It has been shown that 
resistance or susceptibility of mice to several infections, inherited 
as a dominant character, finds its expression on the cellular level 
in a certain fraction of a cell population known as macro- 
phages. These cells, which are present by the million in the 
body of the animals, probably express the most primitive 
“immunological”? defence system of living beings, already 
operative in animals on the lowest level of the vertebrate scale, 
such as hagfish and lampreys, which apparently cannot develop 
serum antibodies to an antigenic stimulus. The macrophage 
defence mechanism has never been adequately investigated 
in human subjects, particularly from the point of view of 
genetic mapping of man based on susceptibility of his macro- 
phages to infectious agents. Neither has the non-homogeneous 
human macrophage population been divided into groups in 
relation to their reaction to infectious agents. Within the 
context of such a study, resistance and susceptibility may be 
discerned as genetic traits in man. Stimulation of the macro- 
phages by certain compounds may result in cell population 
shifts in relation to phagocytic properties which in turn will 
make the host more resistant to pathogenic challenges. This 
has been observed in animals, which may change their resis- 
tance pattern to infections depending on the compound used 
and the time elapsed between its administration and the chal- 
lenge. By predicting this approach, I do not mean to imply that 
we should revert to the application of so-called “‘non-specific 
stimulation”? such as injections of milk or typhoid vaccine, 
methods which have been used commonly by the preceding 
generation and discarded as worthless. On the contrary, it is 
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