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A tumor does not grow freely in its human host as it would in
tissue culture. The host puts up a defense, generically
called “host resistance”, which resembles defense against
infections. There is a homeostatic interaction between the host
and tumor cells or microorganisms based on a dynamic balance
between them and the host microenvironment in which they grow -
an updated version of the seed-and-soil hypothesis. The tumor
arises from an abnormality of growth and differentiation based on
altered structure, regulation and expression of its genes. The
resulting properties of transformation, invasiveness, metastasis,
clonality and heterogeneity give rise to its malignant behavior.
But the outcome of its growth still depends on its interactions
with host defenses for a net result of progression, dormancy or
regression. The process is dynamic and chronic with the balance of
host resistance changing with the advancing stage of tumor growth.
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