Table 3.

Description and Age-Related Changes of Biomarkers of the Immune System

Immune system biomarkers	Brief definition	Function	Age-related changes	Association with disease
Epithelial barriers	Nonimmunological defense (e.g., skin, mucous, coughing)	Prevent pathogens from entering the body	Age-related functional decrease	Compromised barriers associated with an increased risk of pathogenic organism invasion
Complement	A series of plasma proteins that mediate the inflammatory response	Destroys pathogens by attacking their plasma membranes; promotes ingestion of foreign materials by phagocytes	Decreases with age	Lower levels may result in less efficient removal of non-self antigens, which may increase morbidity
Phagocytes	Immune cells that engulf pathogens	Ingest foreign particles (e.g., bacteria); typically first responders that initiate/promote continuation of the inflammatory response	Age-related impairment in the ability to destroy pathogens	Impairments may result in an increased susceptibility to illness and infection
Neutrophils (n)
Granulocytes (g)
Macrophages (m)
Dendritic cells (dc)
C-reactive protein	Protein that increases during systemic inflammation; marker of inflammation	Enhances phagocytosis; assists in the binding of complement to foreign and/or damaged cells	Age-related increases	Higher levels associated with acute illness and increased risk of cardiovascular and other diseases
NK cells	Cytotoxic lymphocytes that destroy compromised host cells	Destroy virally infected cells; inhibit viral reproduction; destroy cancer cells	Many studies show age-related increase in number of NK cells, but a decrease in functionality	Decreased functionality associated with the development and/or progression of infection and disease
T lymphocytes	White blood cells derived from bone marrow that mature in thymus; responsible for cell-mediated immunity	Recognize nonself-antigens—Helper T cells: release cytokines that direct behavior of other immune cells; Regulatory T cells: mediate immune response suppression; Cytotoxic T cells: lysis virally infected cells; inhibit viral reproduction; some become memory cells to fight recurrent infection	More antigen-specific memory T cells with age, but fewer naive T cells; decreased functional capacity; atrophy of the thymus	Decreased number of naive cells and decreased functional capacity results in a reduced ability to mediate an effective response against novel antigens
B lymphocytes	White blood cells derived from bone marrow; responsible for humoral immunity	Recognize nonself-antigens; produce antibodies that neutralize antigens; label infected cells for destruction by phagocytes	Impaired functioning; fewer naive as compared with antigen-specific B cells; decreased antibody production	Impaired functioning results in a diminished response to vaccines and an inefficient response to antigens
Proinflammatory cytokines (e.g., IL-6, TNF-α)	Chemical messengers that influence systemic inflammation	Released by immune cells to promote inflammatory response; affect the differentiation of Helper T cells during an immune response	Most studies find age-related increases	Elevated levels associated with several age-related diseases (e.g., osteoporosis, atherosclerosis)

Note: IL-6 = interleukin 6; NK = natural killer; TNF-α = tumor necrosis factor.