With age, the immune system becomes less effective at protecting the body against pathogens. The adaptive immune system (the part of the immune system that recognises specific pathogens) declines in favour of the innate immune system (which is more involved in inflammation). This results in increased susceptibility to chronic inflammatory diseases. These changes happen for a variety of reasons, including:
Hematopoietic stem cells in the bone marrow decline in number and become less likely to develop into lymphocytes (such as T cells, B cells) and more likely to produce cells of the myeloid lineage (like macrophages and neutrophils).
The thymus, the organ in which T cells mature, shrinks with age. This contributes to a decline in the number of naive T cells (T cells that have not yet encountered foreign antigens and are available to respond to previously unseen pathogens).
Naïve T cells are further depleted by repeated viral infections.
Senescent cells throughout the body release inflammatory molecules that contribute to the overactivity of the innate immune system.
The overall result is that the aging immune system takes longer to respond to pathogens, is less effective at removing them, and is more likely to produce excessive and harmful inflammatory responses.