TABLE 2.

Summary of the key features of age-related immune decline (immunosenescence)¹

Cell type	Effect seen in immunosenescence
T lymphocyte	Decreased numbers in the circulation Imbalances among different phenotypes (e.g., decreased ratio of CD4+ to CD8+ cells) Decline in naive T-lymphocyte production and decreased numbers in the circulation Accumulation of nonfunctional memory T lymphocytes Diminished antigen receptor diversity Impaired responsiveness Impaired proliferation Impaired production of cytokines (e.g., IL-2 and IFN-γ)
B lymphocytes	Decreased numbers of naive B lymphocytes in the circulation Accumulation of nonfunctional memory B lymphocytes in the circulation Impaired responsiveness Altered balance of immunoglobulins
Dendritic cells	Decreased phagocytosis Decreased TLR expression Decreased responsiveness Decreased type 1 IFN production
Neutrophils	Numbers in the circulation are preserved Impaired chemotaxis Impaired oxidative burst and bacterial killing Impaired phagocytosis Decreased TLR expression Decreased production of neutrophil extracellular traps Decreased responsiveness
Monocytes	Altered TLR expression Decreased responsiveness Altered pattern of cytokine production
Macrophages	Impaired phagocytosis Altered TLR expression Increased prostaglandin E2 production
NK cells	Increased numbers in the circulation Imbalances among different phenotypes Impaired cytotoxicity Impaired responsiveness

¹TLR, Toll-like receptor.