What Is IgG p41 Ab and What Does a Positive Result Mean?

IgG p41 Ab refers to an immunoglobulin G antibody that your immune system produces against a specific protein, called p41 (or flagellin B), found on the bacterium that causes Lyme disease. A positive result means your blood contains antibodies that recognize this protein, but it does not, on its own, confirm a Lyme disease diagnosis. The p41 band is one of the earliest and most common antibody responses to appear during a Borrelia burgdorferi infection, yet it is also one of the least specific, frequently lighting up in people who have never been bitten by an infected tick.

What the p41 Protein Actually Is

Borrelia burgdorferi, the spirochete bacterium behind Lyme disease, moves using internal flagella, corkscrew-shaped structures that sit between the bacterium’s inner and outer membranes. The main structural protein of those flagella is a 41-kilodalton molecule called flagellin B, or FlaB. When researchers first cloned and sequenced the gene encoding this protein, they found it shared striking similarities with flagellin proteins from completely unrelated bacteria. The amino-terminal end of Borrelia’s flagellin was roughly 65% similar to flagellin from Bacillus subtilis and Salmonella, with the carboxy-terminal end about 56% similar.¹ That shared architecture is key to understanding why a positive p41 result can be so misleading.

Because flagellin is a large, exposed protein and the immune system encounters it early during infection, antibodies against it tend to be among the first detectable responses. In Lyme serology, this makes p41 a highly sensitive marker: if someone has been infected, their body will almost certainly produce anti-p41 antibodies relatively quickly. The problem is that the conserved regions of the molecule, the parts that look similar across many bacterial species, are the same regions that trigger most of those antibodies. So the immune system of someone who has encountered any number of common bacteria may generate antibodies that happen to bind the p41 flagellin of Borrelia too.

Why a Positive p41 Result Is Not Enough for a Lyme Diagnosis

When your lab report shows a positive IgG p41 band on a Western blot (immunoblot), it means the test detected antibodies in your blood that stuck to the flagellin protein. The catch is that this band appears frequently in people who do not have Lyme disease. In one study that tested 129 control sera from patients with various non-Lyme conditions, about 42% showed IgG reactivity to p41.² Researchers investigating recombinant antigens for Lyme immunoblots reached a similar conclusion years earlier, finding that the complete flagellin protein was “not useful due to a high number of unspecific reactions with control sera.”³

This is why current diagnostic guidelines do not count p41 alone as evidence of Lyme. In the standard testing framework used in the United States, a positive IgG Western blot requires at least five out of ten designated bands to be present. The p41 band is one of those ten, but it carries no more weight than any other individual band. If only p41 lights up and the other bands are negative, the Western blot is interpreted as negative for Lyme disease. The single positive p41 band in that scenario is generally considered a nonspecific finding.

What Can Cause a False-Positive p41 Band

The structural similarity between Borrelia’s flagellin and flagellins from other bacteria is the root cause of most cross-reactivity. Research has mapped which parts of the molecule are responsible. Sera from patients with syphilis, for instance, bind strongly to the conserved amino-terminal domain of the 41 kDa flagellar antigen, which provides a direct structural explanation for why syphilis so often triggers false positives on standard Lyme enzyme immunoassays.⁴ Those syphilis sera did not bind the central, more Borrelia-specific portion of the molecule, which is why some newer tests use only that internal fragment instead of the whole flagellin.

Syphilis is the most dramatic source of cross-reactivity, but it is far from the only one. In the study of non-Lyme control sera mentioned above, patients with syphilis showed the highest rates of IgG and IgM anti-Borrelia bands at about 88%, but patients with HIV antibodies came in at roughly 58%, and patients with antinuclear antibodies (a marker of autoimmune conditions) at about 52%.² Broader reviews of cross-reactivity in Lyme testing have also identified relapsing fever Borrelia species, Epstein-Barr virus, and cytomegalovirus as triggers for nonspecific antibody responses to proteins commonly used in Lyme assays, including FlaB.⁵

In practical terms, this means that if you have ever had mononucleosis, a past syphilis infection (even one that was treated), certain viral infections, or an autoimmune condition, you have a higher-than-average chance of seeing a positive p41 band on a Lyme blot even though Borrelia burgdorferi was never involved.

IgM p41 Versus IgG p41

Lab reports sometimes show p41 results for both IgM and IgG antibodies, and the distinction matters. IgM antibodies are the first wave the immune system sends out after encountering a new pathogen, typically appearing within days to a few weeks. IgG antibodies follow later, usually weeks to months after infection, and tend to persist much longer.

A positive IgM anti-p41 result that is not accompanied by a positive IgG anti-p41 is a common finding in clinical practice, and it is one of the most confusing for patients. A study tracking 63 patients who repeatedly tested positive for IgM anti-p41 antibodies while remaining negative for IgG anti-p41 found that only 10 of those 63 patients had recently experienced erythema migrans, the telltale expanding rash of early Lyme. When the remaining 53 patients, who had little or no clinical evidence of Borrelia infection, were tested with a confirmatory IgM Western blot, only 5 came back positive.⁶ In other words, the vast majority of isolated IgM anti-p41 positives in that group were not caused by active Lyme disease.

This pattern is a well-known pitfall. IgM results are more prone to false positives generally, and the p41 antigen only amplifies that tendency. Clinicians are typically advised not to rely on IgM results beyond the first four to six weeks of suspected illness. After that window, a positive IgM in the absence of IgG raises more suspicion of a nonspecific reaction than of genuine infection.

How Lyme Testing Uses p41 Within the Bigger Picture

Lyme disease testing in the United States has historically followed a two-step process. The first step is a screening test, usually an enzyme immunoassay, that detects a broad signal of anti-Borrelia antibodies. If that screen comes back positive or borderline, a second-tier test, the Western blot, is run to look for antibodies against specific Borrelia proteins. The p41 band is one of several bands evaluated, but the interpretation depends on the total pattern, not any single band.

More recently, labs have been moving toward modified two-tier testing, which replaces the Western blot with a second, different enzyme immunoassay. Studies comparing the two approaches have found that the modified approach tends to be more sensitive for early Lyme disease.⁷ Part of the motivation for this shift is precisely the kind of interpretive challenge that p41 creates: the traditional immunoblot asks clinicians to count individual bands, each with its own specificity quirks, while a second-generation immunoassay can use more carefully selected antigens to reduce false signals. Standard two-tier testing methods rely on immunoblots that have recognized clinical and technical limitations, and modified approaches are being widely adopted as alternatives.⁸

If your lab used a traditional Western blot and the report lists individual bands including p41, the result should be interpreted only in the context of how many other bands are also positive. If your lab used a modified two-tier approach, you may not see individual band calls at all, because the second test produces a single positive-or-negative result rather than a banding pattern.

The Internal Fragment Approach to Flagellin Testing

Because the ends of the flagellin protein are the regions most conserved across bacterial species, researchers have explored whether using only the internal, more Borrelia-specific fragment of the protein could reduce false positives. This internal fragment, sometimes designated p41/i, spans roughly the middle portion of the molecule where the amino acid sequence diverges most from other bacteria’s flagellins.

When assays built around this internal fragment were tested, they performed differently depending on which Borrelia strain the fragment was derived from. Researchers compared fragments from three Borrelia species and found that while whole-cell tests correlated well across strains, the recombinant internal fragment tests showed considerably different results depending on the source strain. The biggest discrepancies appeared between fragments from different Borrelia species, suggesting that immune response to this region is influenced by which strain a patient was actually infected with.⁹ This is mostly relevant in Europe, where multiple Borrelia species circulate and a test designed around one species’ flagellin may miss infections caused by another. In the United States, where Borrelia burgdorferi sensu stricto dominates, strain variation is less of a concern for flagellin-based assays, but it still introduces some variability.

What Happens to p41 Antibodies After Treatment

One question patients often have after seeing a positive p41 band is whether the antibody will go away after treatment. Antibody levels following Lyme disease treatment follow a general pattern: IgM antibodies decline faster than IgG antibodies, but neither class disappears on a reliable schedule. In a follow-up study of patients treated for erythema migrans, positive immunoblot results were found in about 50% before therapy, 57% directly after therapy, and 44% by the end of the follow-up period for IgG. For IgM, the numbers dropped from about 36% before therapy to 43% right after and then down to 12% by the final follow-up.¹⁰

The takeaway is that IgG antibodies, including anti-p41, can persist for months or even years after successful treatment. A positive IgG p41 band months after completing antibiotics does not mean you still have an active infection. It simply means your immune system still carries the memory of having encountered the flagellin protein. This is why Lyme serology is not useful as a “test of cure.” Doctors generally do not repeat blood tests to confirm that treatment worked. Instead, they rely on whether your symptoms have resolved.

When a Positive p41 Band Actually Matters

Despite all its limitations as a standalone marker, p41 is not useless. Its value lies in the context surrounding it. A positive p41 band in combination with four or more other specific IgG bands on a Western blot is genuinely informative, because the pattern of multiple bands together dramatically reduces the chance of a false positive. The more Borrelia-specific proteins your immune system recognizes, the more confident a clinician can be that you were actually infected.

Similarly, an isolated positive p41 on an IgG blot can be meaningful if paired with a strong clinical picture. A patient with a known tick bite in an endemic area, a characteristic expanding rash, and joint swelling who also shows a p41 band on an early blot may be in the early stages of seroconversion, where p41 is the first band to appear and others will follow if the blood is tested again a few weeks later. In that scenario, a doctor might initiate treatment based on clinical judgment rather than waiting for a textbook-perfect five-band blot.

The trouble arises when a positive p41 is the only serologic finding in a patient with vague, nonspecific symptoms and no clear exposure history. In that situation, the p41 band is far more likely to reflect cross-reactivity or a past encounter with some other flagellated bacterium than an active Lyme infection. Anchoring on that single band can lead to unnecessary anxiety, further testing, and sometimes prolonged antibiotic courses that carry their own risks.

Cross-Reactivity Beyond Bacteria

The cross-reactivity problem extends beyond just other spirochetes and flagellated bacteria. Reviews of the Lyme serodiagnosis literature have documented that viral infections can also produce antibodies that bind Borrelia antigens used in standard tests. Both Epstein-Barr virus (the cause of most cases of mononucleosis) and cytomegalovirus have been linked to nonspecific reactivity against proteins like FlaB, OspC, and VlsE.⁵ This is not unique to p41, but because p41 is among the most reactive bands on a blot, it tends to be the one that triggers the initial concern. Patients recovering from mono, for example, sometimes end up being tested for Lyme because of lingering fatigue. If their screening test and p41 band come back positive, it can set off a diagnostic chain that leads away from the actual cause of their symptoms.

Newer Antigens That Are Replacing p41’s Diagnostic Role

Much of the recent progress in Lyme serology has focused on finding antigens that are more specific to Borrelia and less prone to the cross-reactivity problems that plague the whole flagellin molecule. The C6 peptide, derived from the VlsE protein, has become a workhorse in newer assays because antibodies against it are far more specific to Borrelia burgdorferi. Some single-step immunoblot approaches using recombinant protein panels have demonstrated significantly greater sensitivity than FDA-cleared standard two-tier tests for detecting antibodies in early Lyme disease sera.¹¹

These newer approaches are designed to retain the sensitivity that makes flagellin valuable as an early marker while shedding the nonspecific noise. As labs continue to adopt modified two-tier testing and recombinant antigen panels, the traditional Western blot with its individual band calls, including p41, is gradually becoming less central to Lyme diagnosis. That said, many labs still use the older format, so patients and clinicians will continue to encounter p41 results on reports for some time.

Reading Your Lab Report

If you are staring at a lab report that mentions IgG p41 Ab positive, here is how to think about it practically:

Check the overall result: The individual band positivity matters less than the test’s final interpretation. If the Western blot or immunoblot is reported as negative or indeterminate overall, a lone positive p41 band does not change that conclusion.
Count the other bands: If several other IgG bands are also positive and the blot is reported as positive overall, the p41 band is simply one more piece of a supportive pattern.
Consider the clinical context: Were you tested because of a tick bite, a rash, or joint pain in an area where Lyme is common? Or was the test ordered as part of a broad workup for unexplained fatigue? The prior probability of Lyme disease dramatically affects what a positive p41 band means for you.
Ask about the test type: If your lab used a modified two-tier approach, you may not see band-level data at all, and the result is generally more straightforward to interpret.

A positive IgG p41 band is one of those lab findings that sounds alarming in isolation but carries relatively little diagnostic weight on its own. If your doctor tells you the overall blot was negative despite a positive p41, that is a reassuring interpretation, not a missed diagnosis. If you have genuine concerns about Lyme exposure and a single positive p41 is the only finding, a conversation with your doctor about whether retesting in a few weeks makes sense is reasonable, since early seroconversion could produce additional bands over time.