Tumor Immunity
Cancer is a major health problem worldwide and one of the most important causes
of morbidity and mortality in children and adults. The lethality of malignant tumors is
due to their uncontrolled growth within normal tissues, causing damage and
functional impairment. The malignant phenotype of cancers reflects defects in
regulation of cell proliferation, resistance of the tumor cells to apoptotic death,
ability of the tumor cells to invade host tissues and metastasize to distant sites, and
tumor evasion of host immune defense mechanisms. The existence of immune
surveillance has been demonstrated by the increased incidence of some types of
tumors in immunocompromised experimental animals and humans. It is now clear
that the innate and adaptive immune systems do react against many tumors, and
exploiting these reactions to specifically destroy tumors remains an important goal of
tumor immunologists. Several characteristics of tumor antigens and immune
responses to tumors are fundamental to an understanding of tumor immunity and
for the development of strategies for cancer immunotherapy.
The existence of specific anti-tumor immunity implies that tumors must express
antigens that are recognized as foreign by the host. The earliest classification of
tumor antigens was based on their patterns of expression. Antigens that are
expressed on tumor cells but not on normal cells are called tumor-specific antigens;
some of these antigens are unique to individual tumors, whereas others are shared
among tumors of the same type. Tumor antigens that are also expressed on normal
cells are called tumor-associated antigens; in most cases, these antigens are normal
cellular constituents whose expression is aberrant or dys-regulated in tumors. The
modern classification of tumor antigens is based on the molecular structure and
source of antigens expressed by tumor cells that stimulate T cell or antibody
responses in their hosts (Table 1).
