The importance of immunesenescence in the incidence and malignant properties of cancer in hosts of advanced age.

Profound changes in the biology of cancer occur as people and experimental animals age. Many naturally occurring tumors in humans and experimental animals show slower growth with advancing age and longer host survival. The most likely cause for this alteration in tumor biology is the universal phenomenon of cellular immunesenescence. At the onset of sexual maturation, the thymus begins to involute. With this involution, a progressive decline is observed in T-cell functions, e.g., lymphokine production, response to mitogens, and response to antigenic stimulation. It has been proposed that the immune system plays an important role in the stimulation of tumor growth. This role in poorly antigenic tumors may overshadow the role of the immune system as a tumor suppressor. It has been shown that poorly antigenic murine tumors grow more slowly in immune deficient or aged mice. Human tumors are generally poorly antigenic and many (such as lung, breast, and colon carcinomas) also show decreased growth rates in older adults. This article describes age-related changes in the immune system and discusses the theories of immune enhancement of tumor growth. Consideration is also given to the explanation of increased incidence of cancer in elderly patients and the potential role of the immune system in this phenomenon.