The aging of the immune system or immunosenescence is believed to contribute to the increased incidence of cancer seen among older adults. As in so many other areas of medicine, mouse models of cancer are instructive as to how and why this might be.

In the August 2001 issue of Cancer, Immunology and Immunotherapy, researchers at the Hines VA Hospital in Illinois published their observations of lung cancer in aging mice. They noted that CD4+ cells, a line of T cells, declined in number as mice aged. They also noted that in mice with lung cancer, those CD4+ cells were far less likely to produce interferon, a natural anti-cancer substance. Another population of T cells known as CD8+ cells were present in their typical numbers in aging mice, but the CD8+ cells were less able to produce interferon.

A variety of provocative theories have been advanced as to why immunosenescence leads to an increase in tumor formation. Researchers at the University of Wisconsin Primate Research Center have looked at the influence of the aging endocrine (glandular) system. They studied the action of a substance called growth hormone-releasing hormone (GHRH). The sensitivity of cells in various immune cell lines to the action of GHRH appears to decrease as we age, though certain tumor cells were sensitive to GHRH. In addition, the researchers found that the immune systems of postmenopausal women have less sensitivity to GHRH than men's, although women on hormone replacement therapy retain some of that sensitivity. The scientists speculate that GHRH might be an incitement to certain cancers under certain conditions and warrants further study