Immune System & Aging

B-cell senescence encompasses age-related changes in B-cell development, composition, and function that collectively impair humoral immunity. A hallmark is the accumulation of age-associated B cells (ABCs) — a population characterised by expression of T-bet, FcRL5 and CD11c and typically CD21lo/CD27- — which expand preferentially in aging and autoimmune contexts and are poor participants in germinal centre reactions. Germinal centre responses, which are necessary for somatic hypermutation and affinity maturation of antibodies, decline in magnitude and quality with age, resulting in lower-affinity, less class-switched antibody repertoires. These changes contribute to reduced vaccine efficacy and an increased propensity for auto-reactive and low-affinity antibody production in older adults.

The CD4/CD8 ratio is the proportion of CD4+ helper T cells to CD8+ cytotoxic T cells in peripheral blood, with a healthy reference range typically cited as approximately 1.5–2.5. In young, immunocompetent adults, CD4+ cells predominate and coordinate adaptive responses, whereas CD8+ cells patrol for infected or transformed cells. With age — particularly in CMV-seropositive individuals — oligoclonal CD8+ expansions accumulate, compressing the ratio and sometimes inverting it below 1.0, a pattern associated with frailty, poor vaccine responses, and increased all-cause mortality in elderly cohorts and in studies of HIV-positive individuals. An inverted CD4/CD8 ratio is increasingly used as a component of immune-risk profiling in the context of immunosenescence.

Clonal hematopoiesis of indeterminate potential (CHIP) refers to the somatic expansion of a hematopoietic stem cell clone carrying a driver mutation — most commonly in DNMT3A, TET2, ASXL1, or JAK2 — in the blood of individuals without overt haematologic malignancy. The prevalence rises sharply with age, reaching roughly 10–20% in people over 70. Landmark analyses by Jaiswal and colleagues (2017) demonstrated that CHIP carriers have approximately double the risk of cardiovascular events, a finding attributed partly to pro-inflammatory macrophage programming by TET2-mutant clones. Because CHIP alters the epigenetic and transcriptional landscape of blood cells, it confounds DNA-methylation-based biological age clocks and other blood-based aging biomarkers, representing an important caveat in their interpretation.

Cytomegalovirus is a ubiquitous beta-herpesvirus that establishes lifelong latency after primary infection, with seroprevalence ranging from roughly 40–70% in high-income populations, with rates rising substantially in older age groups, to over 90% in parts of Africa, Asia, and South America. Because CMV is never fully cleared, the immune system sustains a large, chronically activated CD8+ T-cell response against viral antigens, a process called memory inflation. Over decades, these oligoclonal CMV-specific CD8+ expansions progressively occupy a growing fraction of the total T-cell repertoire, displacing naive cells and narrowing immunological breadth — a phenomenon linked to poor vaccine responses, frailty, and all-cause mortality in older adults. CMV seropositivity is considered one of the strongest extrinsic drivers of immunosenescence and is now incorporated into immune-age profiling approaches such as the immune risk profile.

The complement system is a network of more than 30 plasma and membrane-bound proteins that constitute an effector arm of innate immunity, activated through three converging pathways — classical (antibody-dependent), lectin, and alternative — all leading to cleavage of C3 and C5 and ultimately the formation of the membrane attack complex (C5b-9). Core functions include opsonisation of pathogens for phagocytosis, direct lysis of gram-negative bacteria and enveloped viruses, recruitment of inflammatory cells via anaphylatoxins C3a and C5a, and clearance of apoptotic debris. With aging, complement dysregulation — including diminished activation capacity alongside paradoxical chronic low-level activation — has been implicated in age-related macular degeneration (AMD), where variants in complement factor H (CFH) substantially modify genetic risk, and in neuroinflammation, where complement-mediated synaptic pruning may become pathologically excessive in conditions such as Alzheimer's disease.

Dendritic cells (DCs) are bone-marrow-derived antigen-presenting cells bridging innate and adaptive immunity. Two main subsets circulate in blood: plasmacytoid DCs (pDCs) detect viral nucleic acids via TLR7 and TLR9 and release type I interferons (IFN-α/β), while conventional myeloid DCs (cDCs) capture antigens, process them into peptides, and present these on MHC class I and II to prime CD8+ cytotoxic and CD4+ helper T cells. With age, pDC numbers decline — Jing et al. (2009) documented fewer circulating pDCs in healthy elderly donors, with reduced IFN-α secretion after influenza stimulation; cDCs remain numerically stable but show impaired antigen uptake and lower co-stimulatory expression (CD40, CD86). Aged DCs exhibit defective IRF-7 upregulation, reduced PI3-kinase activity, and — in respiratory tissue — elevated prostaglandin D2 blunting DC migration to draining lymph nodes during viral infection, impairing T-cell priming (Wong & Goldstein, 2013). Aged DCs also secrete less IL-10, release more pro-inflammatory cytokines, and lose capacity to induce regulatory T cells, driving loss of peripheral tolerance and inflammaging (Agrawal et al., 2017). The result is reduced vaccine immunogenicity; adjuvant approaches such as MF59 are under investigation but remain context-specific.

Interleukin-10 is a pleiotropic anti-inflammatory cytokine produced primarily by macrophages, T regulatory cells, and B cells that restrains pro-inflammatory signalling by inhibiting the synthesis of cytokines including IL-6, TNF-α, and IL-12 in myeloid cells, partly through suppression of NF-κB and STAT1. In healthy immune homeostasis, IL-10 acts as a critical brake on inflammation, preventing immunopathology during infection and autoimmunity; IL-10 acts antagonistically to pro-inflammatory cytokines such as IL-6 and TNF-α. With aging, the balance between IL-10 and pro-inflammatory cytokines is dysregulated in a context-dependent manner, with some studies reporting impaired IL-10 induction in older individuals while others observe elevated levels, suggesting that the relationship is tissue- and stimulus-specific rather than uniformly declining.

IL-1β (Interleukin-1 beta) is a pro-inflammatory cytokine produced by monocytes and macrophages, mediating both acute inflammation and inflammaging — the sterile, low-grade systemic inflammation that accumulates with age. Activation requires two signals: a priming signal (e.g., toll-like receptor engagement) drives transcription of inactive pro-IL-1β; a second signal triggers NLRP3 inflammasome assembly, activating caspase-1, which cleaves pro-IL-1β into its secreted form. In aging tissue, senescent cells release IL-1β as part of the SASP, and declining SIRT2-mediated deacetylation of NLRP3 lowers the assembly threshold, sustaining inflammation. Causal involvement in atherosclerosis was shown in CANTOS (Ridker et al., 2017; n = 10,061): canakinumab, a monoclonal antibody neutralising IL-1β, cut major adverse cardiovascular events by 15% (relative) at 150 mg versus placebo, independent of LDL changes. A pre-specified exploratory safety analysis found dose-dependent reductions in lung cancer incidence. Canakinumab is approved for autoinflammatory syndromes but investigational for cardiovascular indications; colchicine, which inhibits NLRP3 assembly via microtubule disruption, received regulatory approval for cardiovascular secondary prevention in several jurisdictions by 2024.

Immunosenescence is the age-related remodelling of the immune system characterised by a decline in naive lymphocyte output from the thymus and bone marrow, clonal expansion of antigen-experienced memory and effector cells, and a shift in cytokine balance toward a pro-inflammatory baseline. The result is a paradox of concurrent immunodeficiency — reduced capacity to respond to novel pathogens and vaccines — alongside chronic low-grade inflammation (inflammaging). Contributing factors include thymic involution, CMV-driven memory inflation, and accumulating senescent immune cells that secrete SASP components. These changes are associated with increased susceptibility to infections, reduced vaccine efficacy, and higher incidence of autoimmunity and malignancy in older adults.

The M1/M2 framework describes macrophage activation states at two functional extremes: M1 (classically activated) macrophages, induced by IFN-γ and LPS, produce pro-inflammatory mediators including TNF-α, IL-6, IL-12, and reactive oxygen species; M2 (alternatively activated) macrophages, induced principally by IL-4 and IL-13 (and deactivated/suppressed by IL-10), promote tissue repair, phagocytosis of debris, and anti-inflammatory resolution. It is important to note that this binary model is a pedagogical simplification — current transcriptomic and proteomic data support a continuum of macrophage states that do not neatly map onto two poles. With aging, tissue macrophages tend toward a dysregulated inflammatory baseline, impairing resolution of acute inflammation and contributing to the chronic sterile inflammation underlying inflammaging.

Naive T cells are antigen-inexperienced lymphocytes that continuously recirculate through secondary lymphoid organs awaiting their cognate antigen; they are generated in the thymus and depend on tonic TCR and IL-7 signals for peripheral survival. Upon antigen encounter and co-stimulation, they clonally expand and differentiate into effector cells and, subsequently, long-lived memory T cells (central, effector, and tissue-resident subtypes) that enable faster, stronger recall responses. With age, the naive compartment contracts due to thymic involution and homeostatic proliferation-driven phenotypic conversion, while the memory compartment expands — particularly oligoclonal CD8+ populations driven by persistent viral antigens such as CMV. This shift narrows the T-cell receptor repertoire and limits the capacity to respond to novel pathogens or vaccines.

Natural Killer cells are innate lymphoid cells that eliminate virus-infected and malignant cells without prior antigen sensitisation, governed by a balance of activating receptors (NKG2D, NKp46, DNAM-1) and inhibitory receptors that recognise self-MHC class I molecules. NK cells also regulate adaptive immunity through rapid cytokine secretion, particularly IFN-γ. With aging, NK cell numbers in peripheral blood tend to increase but the cells shift toward a terminally differentiated, less proliferative phenotype with reduced cytotoxic capacity per cell and impaired cytokine production. This functional decline contributes to reduced tumour immunosurveillance and poorer control of herpesvirus reactivation in older adults.

Plasma cells are terminally differentiated B cells that have lost expression of B-cell surface markers including BCR-associated CD79 and gained high-level expression of the transcription factors Blimp-1 and IRF4, which drive the factory-like production of secreted antibodies. Long-lived plasma cells occupy specialised survival niches in the bone marrow, sustained by CXCL12/CXCR4 signals, APRIL, and BAFF from stromal cells, and can produce antigen-specific antibodies for decades without re-stimulation. With aging, these bone marrow niches become occupied by long-lived plasma cells from earlier infections and vaccinations, limiting space for newly generated cells — a mechanism contributing to the diminished durability of vaccine responses in elderly individuals, alongside the upstream germinal centre defects of B-cell senescence.

Specialized pro-resolving mediators (SPMs) are endogenous bioactive lipids from polyunsaturated fatty acids: lipoxins arise from arachidonic acid (omega-6), while resolvins, protectins (neuroprotectins), and maresins are enzymatically synthesized from EPA and DHA (omega-3) during resolution of acute inflammation. SPMs actively terminate inflammation: they stimulate macrophage efferocytosis (clearance of apoptotic cells), limit neutrophil recruitment, enhance microbial killing, and promote tissue repair — a distinction Charles Serhan established in his 2014 Nature review defining resolution as a biosynthetically driven process. Resolvin D1 (RvD1) and maresin-1 (MaR1) are measurable in human plasma via liquid chromatography-tandem mass spectrometry (LC-MS/MS) and decline with age as pro-inflammatory eicosanoids rise. Impaired SPM biosynthesis contributes to inflammaging: aged tissues show elevated leukotriene-to-resolvin ratios and defective efferocytosis, accelerating senescent-cell accumulation. In murine aging models, RvD1 rescues efferocytosis defects by preventing MerTK cleavage on macrophages (Rymut et al. 2020). Human evidence from omega-3 supplementation trials remains largely mechanistic; no SPM-based therapeutic has received regulatory approval.

T-cell exhaustion is a hypofunctional state acquired by antigen-specific T lymphocytes — primarily CD8+ cytotoxic T cells — under persistent antigen exposure, such as chronic viral infection (HIV, HBV, HCV) or tumour antigens. Effector functions are lost hierarchically: IL-2 secretion fails first, followed by TNF-α, then IFN-γ; proliferative capacity and cytolytic activity diminish progressively. Exhausted T cells co-express multiple inhibitory checkpoint receptors — PD-1, TIM-3, LAG-3, CTLA-4 — defining the immunophenotype; Wherry et al. (2007, Immunity) mapped these receptors alongside metabolic and TCR-signalling defects in chronic LCMV clone-13 infection. Sustained TCR signalling activates calcineurin/NFAT2, inducing the HMG-box transcription factor TOX; Khan et al. (2019, Nature) showed TOX locks in an exhaustion-specific chromatin landscape through epigenetic remodelling, making the state largely irreversible and only partially rescuable by PD-1/TIM-3 checkpoint blockade. Exhaustion is distinct from T-cell senescence: senescent cells carry permanent replicative arrest with SA-β-gal and p16/p21 upregulation, whereas exhausted T cells retain residual proliferative potential. CMV seropositivity — present in ~40–80% of older adults — drives clonal inflation of Tim-3+PD-1+ T cells accumulating even without active infection (Lee et al., 2016, Aging Cell). In oncology, exhaustion depth in tumour-infiltrating lymphocytes predicts checkpoint-blockade response, since TOX-driven epigenetic fixation limits reinvigoration.

Thymic involution is the progressive replacement of thymic epithelial space by adipose tissue that begins in early childhood and accelerates at puberty, reducing the organ's capacity to generate naive T cells from bone-marrow-derived precursors. By the sixth decade, thymic output of T-cell receptor-diverse naive T cells is diminished to a small fraction of peak adolescent output, constraining the repertoire available to respond to new antigens. Several thymic regeneration strategies have been investigated, including the TRIIM trial by Fahy and colleagues in which a growth hormone/DHEA/metformin combination was associated with partial epigenetic age reversal, and interleukin-7 administration to support peripheral naive T-cell homeostasis and survival; both approaches remain early-stage and require replication in larger trials before clinical conclusions can be drawn. Improving thymic function is considered a plausible route to partially reversing age-related immune decline.

Tumour Necrosis Factor alpha is a pleiotropic pro-inflammatory cytokine produced predominantly by macrophages and monocytes in response to infection or tissue damage, signalling through two receptors — TNFR1 (widely expressed, mediates inflammation and apoptosis) and TNFR2 (expressed mainly on immune and endothelial cells, implicated in cell survival and immunoregulation). At the cellular level, TNF-α activates NF-κB and MAPK pathways, driving expression of adhesion molecules, other cytokines, and acute-phase proteins. Chronically elevated TNF-α is a recognised component of inflammaging, contributing to insulin resistance, muscle wasting (cachexia/sarcopenia), and neuroinflammation. Anti-TNF biologics such as etanercept, infliximab, and adalimumab are established treatments for rheumatoid arthritis and inflammatory bowel disease, and their potential relevance to age-related inflammatory conditions is an active area of translational interest.

Toll-like receptors (TLRs) are ten surface and endosomal pattern recognition receptors forming a primary sensing layer of innate immunity. Cell-surface TLRs (TLR1, TLR2, TLR4, TLR5) detect bacterial lipoproteins, lipopolysaccharide, and flagellin; endosomal TLRs (TLR3, TLR7, TLR8, TLR9) recognise microbial nucleic acids. Each carries a leucine-rich repeat ectodomain and a cytoplasmic TIR domain recruiting MyD88 or TRIF, activating NF-κB, IRF3/7, and MAP kinases to drive TNF-α, IL-1β, IL-6, and type I interferons. Beyond PAMPs, TLRs respond to damage-associated molecular patterns (DAMPs) — HMGB1, oxidised lipids, heat-shock proteins, mislocated self-DNA — released by stressed or dying cells. This sterile DAMP-driven activation is central to inflammaging: the chronic low-grade inflammation linked to cardiovascular disease, neurodegeneration, and increased all-cause mortality in older adults. TLR2 directly governs the senescence-associated secretory phenotype (SASP); Hari et al. (2019, Science Advances) showed TLR2 is markedly induced during oncogene-driven senescence, and its knockdown strongly reduced IL-1β, IL-6, IL-8, and CCL20, affecting over 1,000 downstream genes. TLR4 expression rises in aged innate immune cells; extracellular HSP70/HSP90 acts as an endogenous TLR4 agonist, sustaining NF-κB independently of infection. A miR-146a feedback loop normally restrains TLR/NF-κB output; Olivieri et al. (2013) showed its dysregulation in aged macrophages removes this brake. Human intervention trial evidence is absent as of 2026.

Trained immunity, a concept developed primarily by Mihai Netea and colleagues, refers to the capacity of innate immune cells — principally monocytes, macrophages, and NK cells — to mount enhanced, non-specific responses to secondary stimuli following an initial training event, without invoking classical adaptive immunological memory. The mechanism involves epigenetic reprogramming (histone acetylation and methylation at promoters of inflammatory genes) and metabolic rewiring toward aerobic glycolysis, which together lower the activation threshold of the cell for weeks to months. Well-characterised inducers include β-glucan (a fungal cell-wall component) and the BCG tuberculosis vaccine, which have been shown to reduce heterologous infections and all-cause mortality in some controlled trials. Trained immunity is distinct from the somatic hypermutation and clonal selection that define B- and T-cell memory, and it represents an epigenetic memory in the innate arm of immunity.

T regulatory cells are a specialised CD4+ T-cell subset defined by expression of the transcription factor FOXP3 that suppresses excessive immune responses and maintains self-tolerance. They act through multiple mechanisms including secretion of the anti-inflammatory cytokines IL-10 and TGF-β, competitive consumption of IL-2 via high-affinity CD25 expression, expression of inhibitory co-receptors such as CTLA-4, and direct cytotoxicity toward activated effector cells. In the context of aging, Treg numbers in peripheral blood generally increase or are maintained, but their suppressive function may be impaired; paradoxically, elevated Treg activity has been proposed both to contribute to immunodeficiency by dampening anti-tumour immunity and, in some tissues, to limit chronic inflammation. In the tumour microenvironment, high Treg infiltration is associated with immune evasion and worse prognosis.

Type I interferons — primarily IFN-α (multiple subtypes) and IFN-β — are cytokines secreted by virtually all nucleated cells in response to viral nucleic acids detected by innate pattern recognition receptors, most prominently the cGAS-STING pathway for cytosolic DNA, and TLR7/TLR9 for endosomal RNA/DNA. They signal through the IFNAR1/IFNAR2 receptor complex to induce hundreds of interferon-stimulated genes that establish an antiviral cellular state, activate NK cells, and bridge innate to adaptive immunity. Chronic, low-level type I IFN signalling — driven in aging by accumulated cytosolic DNA fragments, mitochondrial DNA leakage, and retrotransposon activity that activates cGAS-STING — is an important contributor to inflammaging. Pathological extremes of constitutive type I IFN activation are exemplified by SAVI (STING-associated vasculopathy with onset in infancy) and Singleton-Merten syndrome, monogenic interferonopathies that illustrate the tissue-damaging potential of uncontrolled type I IFN signalling.

The vaccine response deteriorates with aging due to multiple converging immunological deficits: reduced naive T- and B-cell diversity, impaired germinal centre reactions, diminished plasma cell longevity, and dysregulated innate sensing all lower the magnitude, affinity, and durability of vaccine-induced immunity. These deficits necessitate formulation adaptations in vaccines targeted at older adults — high-dose or adjuvanted influenza vaccines (e.g., Fluzone High-Dose, Fluad with MF59 adjuvant) elicit substantially stronger seroprotection rates than standard formulations in persons over 65. The recombinant subunit zoster vaccine Shingrix, which uses the AS01B adjuvant system to drive a strong CD4+ T-cell and antibody response, demonstrates approximately 90–97% efficacy against herpes zoster depending on age group (approximately 91% in adults aged 70+, per the pooled ZOE-50/ZOE-70 analysis) compared with roughly 50% for the earlier live-attenuated Zostavax, which was discontinued in the US in 2020, illustrating how adjuvant engineering can partly compensate for age-related immunosenescence.