Hematopoietic stem and progenitor cell populations are responsible for generating immune cells. Their decline is one of the causes of immune failure with age, as the pace at which new immune cells are created falters. There are other equally important issues in immune aging, such as the atrophy of the thymus, where T cells of the adaptive immune system mature, and the accumulation of malfunctioning immune cells in older individuals, but we’ll put those to one side for this discussion.
Stem cell decline with aging is a complicated business with many contributing causes, and the relative importance of those causes seems to differ between populations and tissues. Few stem cell populations are very well studied when it comes to asking why exactly it is that they decline in activity with age. Those that are, such as muscle, hematopoietic, and neural populations all seem to be quite different. Muscle stem cells decline in activity but, given the right signals, appear quite ready to go back to work with minimal signs that they are greatly impacted by damage. Hematopoietic stem cells do appear to be more damage-limited, however.