Blood Biomarkers Are Moving Immuno-Oncology Monitoring Beyond the Biopsy Needle
Analysis of tumor tissue remains the gold standard for cancer diagnosis and biomarker selection, but it captures only a single spatial and temporal slice of a disease that changes under therapeutic pressure. In immuno-oncology, that limitation is especially sharp. Immune infiltration, immune exclusion, clonal evolution, and treatment-induced escape can shift over days to months, while repeat tissue sampling is often invasive, impractical, or impossible.
Peripheral blood offers a different window. It cannot reproduce the architecture of the tumor immune microenvironment, yet it can be sampled repeatedly and can reveal how cancer and host immunity are interacting systemically. The central message of this review is straightforward: liquid biopsy should be viewed as a complement to tissue, not a replacement for it, because blood-based biomarkers can track dynamic immune behavior in ways that static tumor sampling cannot.
The appeal is biological as much as practical. Tumors do not exist in isolation. They release DNA, cells, vesicles, proteins, and other signals into circulation, and they also reshape circulating immune-cell populations and molecular programs. Those signals may expose tumor burden, immune activation, immune suppression, and emerging resistance long before conventional clinical progression becomes obvious.
Circulating Tumor DNA Gives a Dynamic Read on Tumor Burden and Residual Disease
Among the best-studied blood biomarkers, circulating tumor DNA has become the most familiar marker of tumor burden and minimal residual disease. Its value lies in kinetics: falling levels can indicate treatment response, while persistence or reappearance can suggest residual disease or recurrence. In the immunotherapy setting, serial ctDNA assessment may reveal divergence between apparent radiographic stability and underlying molecular progression, especially when immune-related imaging changes muddy the clinical picture.
The biology is not complicated, but it is clinically consequential. ctDNA reflects tumor cell death, shedding, and turnover, so it can function as a molecular counter of active disease. When tumor clones adapt to immune pressure, ctDNA can also expose resistance before lesions enlarge enough to be counted on imaging. Yet interpretation remains difficult because shedding varies widely by tumor type, disease site, burden, vascularity, and technical assay design. A low or undetectable signal does not necessarily mean absence of disease.
This is where assay choice matters. Tumor-informed and tumor-agnostic approaches do not behave identically, and their performance depends on how well the assay can distinguish rare tumor fragments from background cell-free DNA. The review emphasizes that analytic and clinical standardization remain major barriers before ctDNA can be used uniformly as a decision tool across solid tumors.
Circulating Tumor Cells and Immune Cells Reflect Distinct Biology
Circulating tumor cells add a different layer of information. Unlike ctDNA, which reports molecular debris, these cells are intact and may retain phenotypic clues about invasion, epithelial-to-mesenchymal transition, and treatment tolerance. Their rarity makes detection difficult, and their interpretation is complicated by heterogeneity in release, survival in the bloodstream, and assay platforms. Even so, circulating tumor cells can provide cellular information that ctDNA cannot, particularly when investigating metastatic competence or evolving phenotypes under immune selection.
A separate stream of evidence comes from circulating immune cells. Their phenotypes and transcriptomic or proteomic signatures can mirror aspects of the tumor immune microenvironment, although they never fully capture it. Peripheral blood can reveal whether systemic immunity is skewed toward activation, suppression, exhaustion, or myeloid predominance. Those patterns may help explain why some patients respond to checkpoint blockade while others do not, even when tissue-based biomarkers look similar.
One sentence captures the clinical tension: the blood can expose immune drift before the scan does.
Extracellular Vesicles and Plasma Proteins Act as Both Messengers and Markers
The review gives particular attention to extracellular vesicles and plasma proteins because these are not passive bystanders. They can actively shape immunity by transporting nucleic acids, proteins, and lipids between cells, while also serving as measurable signatures of systemic immune state. Their dual role makes them attractive biomarkers and mechanistic mediators at the same time.
This duality is clinically appealing and analytically frustrating. Vesicles and plasma proteins may carry information about immune activation or suppression, but their specificity is limited, and their biological complexity is substantial. Many are influenced by inflammation, tissue injury, or comorbid disease, so a signal that looks cancer-related may not be cancer-specific. Even so, they remain compelling candidates for predicting response to immunotherapy and for identifying immune evasion pathways that tissue biopsy may miss.
Platelets also belong in this group of circulating interpreters. They can interact with tumor cells and immune pathways in ways that influence metastasis and systemic responses, making them another source of biomarker information rather than merely a clotting compartment.
Why Blood Cannot Replace Tumor Tissue
The review is clear that blood-based analysis has an important ceiling. Peripheral markers cannot provide spatial information about the tumor immune microenvironment, where cellular neighborhoods, invasive-front biology, and local suppressive circuits often determine therapeutic response. Tissue remains essential for diagnosis, histologic classification, and many biomarker decisions.
Blood, however, solves a different problem: frequency. Repeated sampling makes it possible to track changes over time without repeated invasive procedures. That repeated access is especially useful in immunotherapy, where resistance can emerge under selective pressure and where early molecular changes may precede overt clinical failure. In practice, this makes liquid biopsy useful for longitudinal surveillance, treatment adaptation, and research into the mechanics of immune escape.
The most realistic model is a paired one. Tissue defines the tumor at a point in time. Blood follows the evolving conversation between tumor and host.
Standardization, Validation, and Clinical Interpretation Remain the Gatekeepers
The field still faces steep implementation barriers. Rigorous validation is needed across platforms, cancer types, and treatment settings. Standardization remains uneven, especially for pre-analytic handling, assay sensitivity, and cross-laboratory reproducibility. Accessibility and cost-effectiveness will also shape whether these approaches remain specialized research tools or become routine components of oncology practice.
Artificial intelligence and multi-omic integration may improve interpretability, but they also raise a familiar problem: more data do not automatically produce better bedside decisions. Clinicians will need tools that are not only sensitive, but explainable and clinically actionable. Without that, blood biomarkers risk becoming sophisticated signals with uncertain meaning.
The review closes on a practical constraint rather than a promise. Liquid biopsy in immuno-oncology is already useful for generating hypotheses, following molecular dynamics, and identifying resistance signals, but its clinical adoption still depends on stronger evidence, tighter standardization, and better integration with tissue-based assessment.