Age-associated changes in the immune system, also known as , are thought to be responsible for the increase in infections and cancer as adults age.

The correct answer is: Immunosenescence.

Explanation:

Immunosenescence can be defined as a biological age-related phenomenon due to which the cells of the immune system are unable to function efficiently with increase in age. As a result of this the aged individuals becomes susceptible to various infections and diseases.This happens because of the following reasons:The immune cells are formed from the progenitor cells called the Hematopoetic Stem Cells (HSCs) which are abundantly found in the bone marrow. These cells have the capacity of self-renewal and also differentiation into various types of immune cells. Due to aging, these cells accumulate large number of Reactive Oxygen Species (ROS) that that causes oxidative damage to the DNA. Also, with age the telomere (the repititive DNA sequence located at the chromosome terminal that protects the end of the chromosome from undergoing recombination or fusion with other chromosomes) length shortens resulting in the generation of damage in DNA. All these reduces the self renewal and differentiation capacity of these cells.Other cells of the immune system like, the white blood cells, anti-body producing B-cells, cytotoxicity inducing T-cells, natural killer cells also accumulate similar DNA degrading components. This reduces their life time and also the efficiency of their activity.