March, 1953]
J.
M.
LALL,
AND
N.
C.
SENA
33
played by the allergic reaction at the site of infection in fixing the invading organisms.
Rich's view is supported by Rirkhaug. Raffel studied the problem from another
angle showed that the chloroform-soluble protein-wax component of the tubercle
bacillus was able to produce allergy to tuberculin in guinea-pigs and although on
the infection of virulent bacilli in these guinea-pigs a modified Koch's phenomenon
could be observed (induration and erythema, but no necrosis), there was no evidence
of acquired resistance. Wilson and Miles have summarized the whole work on the
subject, and to them the experimental and the field evidence suggest that immunity
to tuberculosis is composite, and that allergy is one of the factors which often, though
not always contributes to its establishment. The fixation of the organisms that
occurs at the site of infection in the allergic subject prevents rapid lymphatic dis-
semination and enables the sensitized moni-nuclear cells to ingest and destroy the
invading tubercle bacilli more quickly than in the non-allergic subject.
Degree of induced resistance. The experimental evidence available so far in
guinea-pigs regarding the degree of induced resistance by the vole bacilli and B.C.G.,
points to the conclusion 'that in the early stages of virulent infection in guinea-pigs,
vaccination with the vole bacillus leads to a greater retardation of the disease than
does B.C.G. In the later stages, the degree of resistance is approximately equal.
In cattle according to Griffith and Dalling [1940], the vole bacillus may confer more
resistance than B.C.G.
Stability of virulence. In the early days of B.C.G. vaccination several claims
were put forward that the B.C.G. strain was not a 'virus fixe' and that its virulence
could be increased in vivo and vitro. The controversy centred in those days on the
possibility of a dangerous increase in the virulence of B.C.G. to such an extent that
progressive disease would follow its injection into animals or man. However, since
1930, this question of virulence of the strain has been little discussed and at present
there seems no evidence to prove that organisms of the B.C.G. strain may regain
their virulence in the both and the conclusion that can be safely drawn is that,
whatever, may have occurred in the twenties, since about 1930 it has been impossible
to demonstrate any virulent character of this strain. But one interesting point
worth noting is that almost all the reports of a return of virulence were made
prior to 1930 and since that time there has been a spectacular silence on this subject,
rather concern has now swung in the opposite direction. A fact that emerges from
these results is that in the face of the experimental evidence available, it seems
difficult to support Calmette's contention that the biological properties of B.C.G.
were fixed. Of course, much of the work done to support the presence of certain
virulent elements in the B.C.G. culture had in it errors of observation or of inter-
pretation.
Judging from the history of the evolution of the B.C.G. strain, the enormous
literature that once supported the view that it was capable of regaining its virulence
under certain conditions, the spectacular silence since 1930 over the virulence
question and now the reports of its attenuation, make one feel that the strain se-
parated from its natural habitat for 44 years, is perhaps undergoing slow progressive
attenuation. If so, will not this continued loss of virulence result, also, in lowering
down its protective properties so much so that it may no longer remain an efficient
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