What Does It Mean to Be CD68 Positive?

A tissue sample or cell labeled “CD68 positive” contains a protein called CD68, which is one of the most widely used markers for identifying macrophages, the immune cells that patrol your body by engulfing debris, pathogens, and dead cells. When a pathologist reports CD68 positivity, they are usually confirming that macrophages are present in the tissue being examined. But the clinical meaning of that finding depends heavily on context, because macrophage infiltration tells a different story in a tumor biopsy than it does in a joint sample or a brain autopsy.

The Protein Itself

CD68 is a glycoprotein that belongs to a family of molecules lining the membranes of lysosomes, the compartments inside cells that break down waste. It sits primarily inside the cell rather than on its surface, which is why pathologists typically need to use techniques that penetrate the cell membrane to detect it. The protein carries heavy sugar chains, including a mucin-like domain covered in sialic acid residues, and its glycosylation pattern changes when macrophages actively engulf material.¹ Because it lives in lysosomes and ramps up when cells are busy digesting things, CD68 is commonly considered a marker of activated, phagocytic macrophages rather than just a flag for any macrophage sitting quietly in tissue.

In the brain, CD68 marks microglia, the resident immune cells of the central nervous system. Resting microglia express some CD68, but levels jump when these cells shift into an active state. A systematic review of post-mortem brain studies described CD68 as labeling the lysosome and therefore primarily reflecting activated phagocytic microglia, not simply the total number of microglia present.² This distinction matters when interpreting results from neurological studies, where knowing whether the brain’s immune cells are merely present versus actively inflamed can change the clinical picture.

How Pathologists Use CD68 Staining

In a pathology lab, CD68 positivity is detected through immunohistochemistry, a technique that uses antibodies to latch onto the CD68 protein in a tissue slice and produce a visible stain. Several different antibody clones target CD68, and they do not all behave the same way. The three most common clones, known as KP1, EBM11, and PGM1, vary in how specifically they mark macrophages versus other cell types. One study testing these clones on fibroblasts found that nearly all fibroblasts stained positive with KP1 and EBM11, while only a small percentage stained with PGM1.³ In other words, two of the most popular clones can flag cells that are not macrophages at all. PGM1 tends to be more selective, but it is also less sensitive. This trade-off between sensitivity and specificity means that a pathologist’s choice of antibody clone can influence whether a sample comes back “CD68 positive” and how much weight that label carries.

The fixation method used to preserve tissue before staining adds another layer of complexity. Some fixation approaches can mask or destroy CD68 epitopes, meaning the antibody has nothing to grab onto. Research comparing different preparation methods found that CD68 antigenicity could not be reliably recovered from paraffin-embedded sections regardless of the antigen retrieval technique used.⁴ Similarly, the type of fixative matters. Standard neutral buffered formalin and zinc formalin preserved CD68 staining significantly better than Bouin’s fluid, which essentially wiped it out.⁵ A false-negative CD68 result can happen if the tissue was not handled optimally before staining, which is one reason pathologists interpret these results alongside clinical context rather than in isolation.

CD68 Positivity in Cancer Biopsies

When a cancer biopsy comes back showing CD68-positive cells, the pathologist is identifying tumor-associated macrophages, or TAMs, inside or around the tumor. Macrophages are drawn to tumors by chemical signals, and once there, they can either help fight the cancer or, paradoxically, be co-opted by it to support tumor growth, supply blood vessels, and suppress other immune responses. A high density of CD68-positive TAMs in a tumor is generally an unfavorable sign. In one study of esophageal cancer, patients with heavy CD68-positive macrophage infiltration in the tumor had shorter overall survival, and these macrophages were linked to higher expression of an enzyme involved in tumor invasion and spread.⁶

Breast cancer data tells a similar story. A systematic review and meta-analysis of breast cancer outcomes found that a high density of TAMs was associated with roughly 70 percent higher risk of death and about 64 percent higher risk of disease progression compared to patients with fewer TAMs.⁷ But CD68 alone does not tell the whole story, because it marks all macrophages regardless of whether they are trying to fight the tumor or helping it grow.

Why CD163 Often Matters More Than CD68 in Tumors

Macrophages are not a single uniform cell type. Broadly, they exist on a spectrum from pro-inflammatory cells that attack threats to anti-inflammatory cells that promote tissue repair and, in the tumor context, can help cancer evade the immune system. CD68 marks macrophages across this entire spectrum, which is why it is called a “pan-macrophage” marker. CD163, by contrast, preferentially labels the anti-inflammatory end of the spectrum, the macrophages more commonly associated with tumor promotion.

This distinction has real prognostic consequences. The same breast cancer meta-analysis found that high CD163-positive macrophage density was associated with more than double the risk of death, whereas high CD68-positive density carried about a 50 percent increase in risk. The difference between these two markers was statistically significant.⁷ A meta-analysis of head and neck cancers reached a similar conclusion: while both CD68 and CD163 had prognostic value for overall survival, CD163 was the stronger predictor.⁸

This is why many oncology pathology reports now include both markers. A biopsy showing lots of CD68-positive but few CD163-positive macrophages paints a very different picture from one dominated by CD163. The ratio between these populations gives oncologists a better sense of how the tumor’s immune environment is tilted. Immunotherapy research is actively targeting this balance. In triple-negative breast cancer trials, for example, treatment combinations including checkpoint inhibitors and a drug called eganelisib were shown to shift the macrophage population toward a more activated, tumor-fighting state in paired biopsies taken before and after treatment.⁹ Separate preclinical work on blocking the extracellular protein HMGB1 showed a similar effect, shifting the macrophage balance without changing the overall number of immune cells.¹⁰

CD68 in the Brain

In neurological research and neuropathology, CD68 has become a go-to marker for understanding whether the brain’s resident immune cells are in an active inflammatory state. A systematic review of post-mortem Alzheimer’s disease studies found that 17 of the identified studies reported increased CD68 expression or cell counts in Alzheimer’s brains compared to controls in at least one region. The review concluded that the apparent increases in microglial markers in Alzheimer’s disease reflect increased activation rather than a greater absolute number of microglia.² Another study found that the presence of dementia was positively associated with CD68 levels, and among participants without dementia, higher cognitive scores on standard testing were associated with lower CD68 levels.¹¹

CD68 is not limited to Alzheimer’s research. In amyotrophic lateral sclerosis, the progressive motor neuron disease, investigators found significantly increased microglial CD68 expression in the motor cortex of ALS patients, with CD68 levels correlating with the burden of a pathological protein called phosphorylated TDP-43. In a mouse model of the disease, CD68 elevation appeared early, even before other inflammatory markers rose, suggesting it could serve as an early indicator of the neuroinflammatory process.¹²

Tracking Treatment Response in Rheumatoid Arthritis

One of the most clinically validated uses of CD68 is in rheumatoid arthritis, where it serves as a biomarker for whether a treatment is actually working at the tissue level. In rheumatoid joints, the synovium (the membrane lining the joint) becomes infiltrated with macrophages that drive inflammation and cartilage destruction. CD68-positive macrophages in the sublining layer of the synovium, just below the joint surface, have been shown to closely track disease activity.

A multi-center validation study found a strong correlation between changes in sublining CD68 counts and changes in the standard clinical disease activity score, with a Pearson correlation of 0.895.¹³ In practical terms, patients who responded well to treatment showed a large drop in sublining macrophages, moderate responders showed a smaller drop, and non-responders showed barely any change. In fact, the change in sublining macrophages explained about 76 percent of the variation in clinical improvement.¹⁴ This makes CD68 sublining counts one of the most reliable tissue-level biomarkers available in rheumatology, and it is increasingly used in clinical trials to evaluate new drugs.

CD68 in Kidney Transplant Biopsies

When a transplanted kidney starts to fail, one of the key questions is whether the body’s immune system is rejecting the organ. Pathologists examine biopsies for various immune cell markers, and CD68-positive macrophage infiltration has emerged as a meaningful finding. CD68 counts are significantly elevated in biopsies showing rejection compared to biopsies without rejection.¹⁵ More granular analysis has shown that the pattern of macrophage infiltration differs depending on the type of rejection. In T-cell-mediated rejection, CD68-positive cells concentrate around the kidney’s tubules and blood vessels, while in antibody-mediated rejection, they cluster primarily around the tubules.¹⁶ The density and distribution of these macrophages help transplant teams decide how aggressively to treat rejection and gauge how well the organ will function going forward.

Granulomas and Inflammatory Diseases

Granulomas, the tight clusters of immune cells that form in diseases like tuberculosis and sarcoidosis, are built primarily from macrophages that have transformed into specialized epithelioid cells and sometimes fused into giant cells. All of these cell types stain positive for CD68. In lung and lymph node tissue from both tuberculosis and sarcoidosis patients, CD68 labeled alveolar macrophages, interstitial macrophages, epithelioid macrophages, and giant cells, functioning as a reliable pan-macrophage marker across these conditions.¹⁷ A study comparing granulomas across several diseases, including common variable immunodeficiency, confirmed that CD68 expression was present in granulomas regardless of the underlying cause.¹⁸

This universality is both CD68’s strength and its limitation in granulomatous disease. It confirms that the cells forming the granuloma are macrophage-derived, but it cannot distinguish tuberculosis granulomas from sarcoidosis granulomas or from those caused by fungal infections. Pathologists rely on additional markers, staining for infectious organisms, and clinical context to make that distinction.

CD68 and Atherosclerosis

Inside artery walls, macrophages play a central role in the development of atherosclerotic plaques. When macrophages take up oxidized cholesterol particles, they balloon into lipid-laden “foam cells” that accumulate and form the fatty core of a plaque.¹⁹ CD68 staining of arterial tissue is one of the standard ways to visualize and quantify macrophage infiltration in plaques. In cardiovascular research, the amount of CD68-positive staining in a plaque is used as a proxy for how inflamed and potentially unstable that plaque is. Plaques heavily infiltrated with macrophages are more prone to rupture, which is the event that triggers most heart attacks and strokes.

When CD68 Positivity Is Misleading

Perhaps the most important thing to understand about CD68 is that it is not perfectly specific to macrophages. Several other cell types can express it, and mistaking those cells for macrophages can lead to diagnostic errors. The most striking example involves melanoma. A study examining melanoma samples found that 86 percent of melanoma tumors showed CD68 immunoreactivity when more than 5 percent of tumor cells stained positive, and the staining pattern was typically diffuse but weak.²⁰ This means a pathologist evaluating a poorly differentiated tumor could potentially mistake melanoma cells for macrophages if relying on CD68 alone. Case reports have documented exactly this scenario, with amelanotic melanoma (melanoma lacking its characteristic dark pigment) being misdiagnosed because of aberrant CD68 expression.²¹

As mentioned earlier, fibroblasts also stain positive with certain CD68 antibody clones, and various other cell types including some dendritic cells and certain tumor cells can express the protein. This is why pathology reports rarely hinge on a single marker. CD68 positivity is interpreted alongside a panel of other stains, and the morphology of the cells under the microscope matters as much as whether they light up with the antibody. A cell that is CD68 positive, S100 negative, and HMB45 negative is more likely a macrophage. A cell that is CD68 positive but also S100 positive and HMB45 positive is almost certainly a melanoma cell, not a macrophage at all.

CD68 in Infectious Disease Research

Macrophages are among the first immune cells to encounter invading pathogens, and their behavior in chronic infections has drawn intense interest. In HIV and SIV (the simian version of HIV), brain macrophages have been identified as a potential latent reservoir for the virus, meaning they can harbor dormant virus even when antiviral therapy has suppressed the infection elsewhere in the body. Researchers used CD68 alongside other markers to identify and quantify these macrophage reservoirs in the brains of SIV-infected primates receiving antiretroviral therapy, demonstrating that replication-competent virus persisted in brain macrophages despite suppressive treatment.²² This finding has implications for HIV cure research, since eliminating the virus from macrophage reservoirs may be necessary on top of clearing it from T cells.

In tuberculosis, CD68 staining helps researchers characterize the macrophage populations within granulomas and understand how effectively infected macrophages are containing or failing to contain the bacteria. The staining pattern, combined with markers for activation and scavenger receptors, reveals whether macrophages in a tuberculous granuloma are actively phagocytic or have shifted to a more quiescent state that might allow bacterial persistence.

Reading a Pathology Report That Mentions CD68

If you are a patient or caregiver looking at a pathology report, CD68-positive staining by itself is neither good news nor bad news. It simply means macrophages were identified in the sample. The context tells you what that means. In a joint biopsy for rheumatoid arthritis, abundant CD68-positive cells in the sublining layer suggest active disease. In a tumor biopsy, a high density of CD68-positive cells points to a macrophage-rich tumor environment, which often (though not always) correlates with a worse prognosis, particularly when paired with high CD163 staining. In a kidney transplant biopsy, elevated CD68 counts raise concern about rejection. And in a skin or lymph node biopsy of an unusual tumor, the pathologist will be careful to confirm that CD68-positive cells are actually macrophages and not tumor cells with aberrant expression.

Pathologists are trained to read these results as one piece of a larger puzzle. No diagnosis should ever rest on a single immunohistochemical stain. If your report mentions CD68 and you are uncertain what it means for your specific condition, the most productive step is to ask the treating physician how the finding fits into the overall diagnostic picture, because the same stain result carries different weight in different clinical scenarios.