Every blood cell in your body traces back to a single pool of stem cells in the bone marrow, and those stem cells face a fundamental fork in the road: become a myeloid cell or become a lymphoid cell. Myeloid cells handle the rapid, broad-spectrum side of immunity and carry oxygen and clot wounds, while lymphoid cells run the slower, precision-guided arm that remembers specific threats. This distinction matters far beyond a biology textbook, because as people age, the stem cells that feed both branches start to accumulate genetic mutations and copy themselves unevenly, a phenomenon called clonal hematopoiesis. Understanding how myeloid and lymphoid lineages differ is the key to understanding why clonal hematopoiesis skews toward myeloid output and why that skew raises the risk of heart disease, blood cancers, and chronic inflammation.
What Myeloid and Lymphoid Cells Actually Do
Myeloid cells are the workhorses of innate immunity, the body’s first line of defense. The category includes neutrophils (the most abundant white blood cells, which swarm to infection sites), monocytes and macrophages (which engulf debris and pathogens), eosinophils and basophils (involved in allergic responses and parasite defense), and dendritic cells (which capture foreign material and present it to other immune cells). Red blood cells and platelets also descend from the myeloid branch. In short, myeloid cells keep you alive minute to minute: they deliver oxygen, seal wounds, and attack invaders on contact. They are crucial effectors of the innate immune response and also help shape the adaptive immune response that lymphoid cells carry out.1Nature Reviews Immunology. Epigenetic control of myeloid cell differentiation, identity and function
Lymphoid cells operate on a different timescale. T cells and B cells, the major lymphoid populations, mount responses that are tailored to a specific pathogen. B cells produce antibodies. T cells either coordinate the immune attack or directly kill infected cells. A third group, natural killer (NK) cells, bridges the two worlds: they are classified as lymphoid but act more like innate defenders, killing virus-infected cells without needing prior exposure. The defining feature of the lymphoid branch is memory. After a first encounter with a pathogen, long-lived memory T and B cells persist for years, enabling a faster and stronger response if that same threat reappears.2PubMed. Adaptive immunity This is why vaccines work: they train the lymphoid arm to remember a threat it has never fully faced.
How the Two Lineages Split
Hematopoietic stem cells sit at the top of a hierarchy. They are self-renewing, meaning they can copy themselves indefinitely, and they are multipotent, meaning they can give rise to any blood cell type. Differentiation proceeds through a series of restriction steps in which a cell progressively loses the ability to become other cell types.3PubMed Central. Lymphoid and myeloid lineage commitment in multipotent hematopoietic progenitors A stem cell first becomes a multipotent progenitor, then commits to either the myeloid or lymphoid path. That commitment is driven by a complex regulatory network of transcription factors, signaling molecules, and chromatin-remodeling proteins.4PubMed Central. Logical modeling of lymphoid and myeloid cell specification and transdifferentiation
The split is not as clean as older textbook diagrams suggest. Research has shown that some progenitor cells retain the ability to produce both B cells, T cells, and certain myeloid cells (like granulocytes and macrophages) while losing the capacity to make red blood cells and platelets. In one study, progenitor cells that expressed high levels of a surface receptor called Flt3 efficiently generated myeloid granulocyte-macrophage progeny and lymphoid cells, but fewer than 3% produced megakaryocytes (platelet precursors) and virtually none made red blood cell precursors.5Cell. The earliest branchpoint in adult hematopoiesis can be identified by conditional expression of Flt3 In addition to transcription factors, microRNAs also help steer these fate decisions, adding another layer of regulation that researchers are still mapping.6PubMed. Gene regulatory networks directing myeloid and lymphoid cell fates within the immune system
The Aging Myeloid Shift
One of the most consistent findings in blood biology is that the immune system tilts toward myeloid output as people get older. In aged bone marrow, the proportion of myeloid cells rises while lymphoid cells, particularly B cells and naive T cells, decline. This is not just about having more of one cell type; it reflects a change in the stem cells themselves. Aged human stem cells are more numerous, less dormant, and more likely to produce myeloid offspring than young stem cells. Gene expression studies confirm that aged stem cells ramp up genes associated with the myeloid lineage and even with myeloid cancers.7PubMed Central. Human bone marrow hematopoietic stem cells are increased in frequency and myeloid-biased with age
This myeloid bias has real consequences. The decline in lymphoid progenitors means fewer fresh naive T and B cells, which weakens the body’s ability to mount targeted immune responses to new infections and vaccines. That is a big part of why older adults are more vulnerable to influenza, pneumonia, and other infections. Meanwhile, the expanding myeloid compartment pumps out more inflammatory signals, contributing to the low-grade, chronic inflammation that characterizes aging.
A striking mouse experiment showed that this shift can be partially reversed. Researchers used antibodies to selectively deplete myeloid-biased stem cells in aged mice. The result was a measurable rebound in lymphoid progenitors, naive T cells, and B cells, along with a reduction in markers of immune decline.8Nature. Depleting myeloid-biased haematopoietic stem cells rejuvenates aged immunity Recent work has identified a protein called clusterin as one molecular driver of this myeloid tilt, linking it to changes in mitochondrial function within aged stem cells.9Nature Aging. Clusterin drives myeloid bias in aged hematopoietic stem cells by regulating mitochondrial function
Clonal Hematopoiesis and Why the Myeloid-Lymphoid Distinction Matters
Clonal hematopoiesis of indeterminate potential, often shortened to CHIP, describes a situation in which one stem cell acquires a genetic mutation and then copies itself so effectively that its descendants make up a detectable fraction of a person’s blood cells, even though the person has no blood cancer or other disease. The prevalence climbs steeply with age: roughly one in ten people between 70 and 80 carry a detectable clone.10PubMed Central. Clonal Hematopoiesis of Indeterminate Potential At younger ages the rate is much lower, but more sensitive sequencing methods suggest that very small clones are present in most healthy adults if you look hard enough.11PubMed Central. Clonal haematopoiesis harbouring AML-associated mutations is ubiquitous in healthy adults
CHIP is not cancer. Over an eight-year follow-up period, only about 4% of people with detectable clones developed a blood cancer, fewer than 1% per year.12Best Practice & Research Clinical Haematology. How predictive is the finding of clonal hematopoiesis for the development of myelodysplastic syndromes (MDS) or acute myeloid leukemia (AML)? But it is also not nothing. The mutations that drive clonal hematopoiesis are overwhelmingly linked to genes that steer myeloid development, and the clones tend to push blood production further in a myeloid direction. That myeloid skew is what connects the basic biology of the two lineages to a growing list of age-related health risks.
The Most Common Driver Mutations
Two genes dominate the landscape of clonal hematopoiesis: DNMT3A and TET2. Both encode enzymes involved in regulating how genes are read, specifically by adding or removing chemical tags on DNA. When either gene is mutated, the affected stem cell gains a competitive advantage that spurs clonal expansion and skews its output toward myeloid cells.13PubMed Central. Clonal hematopoiesis driven by DNMT3A and TET2 mutations: role in monocyte and macrophage biology and atherosclerotic cardiovascular disease Single-cell studies of human bone marrow samples have clarified how this works: most of the selective advantage occurs at the stem cell level, and TET2 mutations in particular accelerate myeloid maturation in a dose-dependent way, meaning that carrying two mutant copies pushes even harder toward myeloid output than one.14PubMed Central. Selective advantage of mutant stem cells in human clonal hematopoiesis is associated with attenuated response to inflammation and aging
Why do mutant stem cells win the competition? Recent evidence supports a counterintuitive model. Both mutant and non-mutant stem cells in people with CHIP show elevated inflammatory and aging-related gene signatures compared with stem cells from people without CHIP. But the mutant stem cells appear to be relatively resistant to the damaging effects of that inflammation, giving them a survival edge in an increasingly hostile bone marrow environment.14PubMed Central. Selective advantage of mutant stem cells in human clonal hematopoiesis is associated with attenuated response to inflammation and aging In other words, inflammation does not just follow CHIP; it may help cause it by creating conditions that favor the mutant clone.
Beyond DNMT3A and TET2, mutations in splicing factor genes such as SF3B1 and SRSF2 also contribute to clonal expansion, though they appear to work somewhat differently. Mouse models suggest that splicing factor mutations on their own do not necessarily give stem cells a strong growth advantage. Instead, they seem to amplify the effects of co-occurring mutations in genes like TET2, promoting clonal expansion and, in some cases, progression toward leukemia through a synergistic interplay.15PubMed. The Impact of Splicing Factor Mutations on Clonal Hematopoiesis and Myeloid Neoplasm Progression
The Cardiovascular Connection
The health risk that has generated the most attention around CHIP is not blood cancer but cardiovascular disease. Mutant myeloid cells, especially monocytes and macrophages derived from DNMT3A- and TET2-mutant clones, produce elevated levels of inflammatory molecules like IL-1β and IL-6. These signals promote a pro-atherogenic environment, meaning they accelerate plaque buildup in arteries and contribute to plaque instability, raising the risk of heart attack and stroke.16PubMed Central. Clonal hematopoiesis at the crossroads of Inflammaging and cardiovascular disease: Mechanistic insights and translational horizons
Not all CHIP mutations carry the same cardiovascular risk. Experimental evidence shows that TET2 and JAK2 mutations are more atherogenic than DNMT3A mutations.17PubMed Central. Clonal hematopoiesis and atherosclerosis Mouse studies have pinpointed part of the reason: loss of either TET2 or DNMT3A expands the myeloid compartment and generates a distinct macrophage population in arterial plaques that expresses high levels of several inflammatory mediators.18Blood. Loss-of-Function Mutations in Dnmt3a and Tet2 Lead to Accelerated Atherosclerosis and Convergent Macrophage Phenotypes in Mice In that study, mice receiving even a small fraction of DNMT3A-deficient bone marrow developed arterial lesions about 40% larger than controls.
The inflammatory pathway in TET2-mutant CHIP has been traced in some detail. TET2-deficient macrophages show heightened activation of the NLRP3 inflammasome, a molecular alarm system that drives IL-1β production. Cholesterol loading makes this worse: the combination of TET2 loss and high cholesterol acts synergistically to amplify inflammasome activation, and this has been confirmed in human TET2-deficient macrophages as well as in mice.19PubMed Central. BRCC3-Mediated NLRP3 Deubiquitylation Promotes Inflammasome Activation and Atherosclerosis in Tet2 Clonal Hematopoiesis The same inflammasome pathway appears to contribute to hypertension: in a mouse model, TET2 loss in blood cells led to sodium retention and elevated blood pressure, effects that were reversed by an NLRP3 inhibitor.20PubMed Central. Experimental TET2 Clonal Hematopoiesis Predisposes to Renal Hypertension Through an Inflammasome-Mediated Mechanism
The Bone Marrow Environment Is Not a Bystander
Researchers used to think of clonal hematopoiesis as a purely cell-autonomous problem: a stem cell picks up a mutation, gains a growth edge, and expands. The bone marrow environment around it was considered passive. That view is changing. Studies of human bone marrow samples from people with CHIP and from patients with myelodysplastic syndromes (MDS, a pre-leukemic condition) show that the bone marrow niche itself is inflamed, with remodeled stromal cells that differ from those in healthy marrow.21PubMed Central. Inflammatory stromal and T cells mediate human bone marrow niche remodeling in clonal hematopoiesis and myelodysplasia
The emerging picture is a feedback loop. A pre-existing inflammatory microenvironment in the marrow may act as a selective force, favoring the expansion of mutant stem cells that tolerate inflammation better. Those mutant cells then produce myeloid offspring that generate even more inflammatory signals, further remodeling the niche. In CHIP, the mutant stem cells still retain the ability to stimulate supportive stromal signals. But in MDS, that capacity breaks down, and the niche becomes dysfunctional, contributing to the poor blood production that defines the disease.21PubMed Central. Inflammatory stromal and T cells mediate human bone marrow niche remodeling in clonal hematopoiesis and myelodysplasia
Lymphoid Clonal Hematopoiesis Exists Too
Most conversations about clonal hematopoiesis focus on myeloid-associated mutations, but a lymphoid counterpart also exists. Researchers analyzing sequencing data from over 46,000 individuals distinguished between myeloid CHIP (M-CHIP) and lymphoid CHIP (L-CHIP) based on whether the mutated genes are associated with myeloid or lymphoid cancers. L-CHIP turned out to be considerably less common, found in about 1.3% of the study population compared with 5.8% for M-CHIP. Its mutations were spread more evenly across a larger number of genes rather than clustering in just a few. L-CHIP was associated with an increased risk of chronic lymphocytic leukemia and small lymphocytic lymphoma but not with myeloid cancers, and conversely, M-CHIP predicted myeloid malignancy but not lymphoid malignancy.22Nature Publishing Group. Lymphoid clonal hematopoiesis: implications for malignancy, immunity, and treatment
This distinction has practical implications. The cardiovascular and inflammatory risks that dominate CHIP discussions are driven mainly by myeloid-biased clones and their inflammatory myeloid offspring. Whether L-CHIP carries comparable non-cancer health risks is still an open question, and the research is much thinner. For the moment, the myeloid side of clonal hematopoiesis remains the bigger clinical concern, both because it is more common and because its downstream effects on inflammation and heart disease are better established.
When Cancer Treatment Accelerates Clonal Expansion
Age is the biggest driver of clonal hematopoiesis, but it is not the only one. Chemotherapy and radiation therapy place enormous selective pressure on stem cells, killing off normal cells and creating an opening for mutant clones that can tolerate the damage. Conventional cytotoxic therapies are known to select for TP53-mutant clones, which carry a high risk for progression to therapy-related myelodysplastic neoplasms.23PubMed Central. Fitness Landscape of Clonal Hematopoiesis Under Selective Pressure of Immune Checkpoint Blockade
A study tracking mutations before and after cancer treatment found more than a twofold increase in detectable TP53 mutations between pretreatment and last follow-up. After chemoradiation, 38% of TP53 mutations grew in clone size while only 5% shrank, a pattern significantly more pronounced than for other genes combined.24PubMed Central. Impact of cancer therapy on clonal hematopoiesis mutations and subsequent clinical outcomes This means cancer survivors may carry expanded clones that put them at elevated risk for secondary blood cancers years down the line. The mutational profile of therapy-related clonal hematopoiesis also looks different from the age-related version: work in long-term survivors of pediatric cancer found that the mutation spectrum in therapy-related clones was distinct from the age-related pattern, which in healthy controls is dominated by a particular type of base change (C-to-T transitions).25PubMed Central. Dynamics of Age- versus Therapy-Related Clonal Hematopoiesis in Long-term Survivors of Pediatric Cancer
Detecting Clonal Hematopoiesis
Standard clinical sequencing typically picks up mutations present in about 2% or more of blood cells. Specialized error-corrected sequencing methods push that limit far lower. One validated approach achieved a detection threshold of 0.4% at high read depths, with perfect sensitivity and specificity against reference standards.26PubMed Central. Error-corrected ultradeep next-generation sequencing for detection of clonal haematopoiesis and haematological neoplasms – sensitivity, specificity and accuracy Even more sensitive research-grade methods can detect clones as rare as 0.03% of cells, which is why studies using those tools find clonal hematopoiesis in the majority of healthy adults.11PubMed Central. Clonal haematopoiesis harbouring AML-associated mutations is ubiquitous in healthy adults
This creates a clinical gray area. A clone that makes up 0.05% of your blood cells is biologically real, but no one knows whether it matters for your health. The clinical definition of CHIP generally requires a variant allele frequency of at least 2%, a threshold chosen partly because that is where the risk data are strongest. Below that, you are in a zone where the mutation is detectable but its significance is genuinely unknown. This ambiguity feeds into a broader question about whether widespread screening for CHIP would help people or just worry them.
Emerging Treatments and the Question of Screening
Because CHIP’s main health threat appears to flow through inflammation rather than direct malignant transformation, anti-inflammatory therapies are a logical target. Researchers are exploring several categories of intervention, including drugs that block the NLRP3 inflammasome, IL-1β inhibitors, epigenetic modulators, and metabolic inhibitors aimed at the specific pathways that mutant myeloid cells exploit.27PubMed Central. Novel and emerging therapeutic strategies for clonal hematopoiesis None of these are yet standard of care for CHIP, but clinical trials are underway, particularly for people who carry high-risk mutations like TET2 or JAK2 and already have cardiovascular disease.
The screening question is thornier. A survey of people who learned about CHIP found that nearly 30% experienced moderate or greater anxiety, and about 4% reported severe or very severe anxiety. Those with the most anxiety were significantly less likely to want testing in the first place.28PubMed Central. Patient perspectives on testing for clonal hematopoiesis of indeterminate potential Given that the yearly risk of progressing to blood cancer is under 1%, and that no proven preventive therapy yet exists for most people with CHIP, population-wide screening could easily generate more worry than benefit. For now, CHIP is most often discovered incidentally, when blood is sequenced for other reasons, such as during a cancer workup or a research study. The challenge going forward is figuring out which people with CHIP genuinely need monitoring and intervention, and which can safely be told that their clone is, for all practical purposes, a normal part of aging.