41 kD IgG Band Reactive Explained: Lyme, Lupus & False Positives

If you are looking at a lab report showing "41 kD IgG: Reactive" or "IgG P41 Ab: Present" and wondering whether you have Lyme disease — the short answer is: this single band result does not confirm Lyme disease, and it is one of the most commonly misunderstood findings in Lyme testing.

A reactive 41 kD IgG band — also reported as "IgG P41 Ab: Present" — does not diagnose Lyme disease on its own. The 41 kD band is the least specific band on the Lyme IgG Western blot and commonly appears in people with lupus, Epstein-Barr virus, syphilis, or even normal oral bacteria — without any Borrelia burgdorferi exposure. CDC surveillance criteria define a positive IgG Western blot as 5 of 10 specific IgG bands reactive. One band is not a positive test.

AT A GLANCE

• The 41 kD IgG band (also called IgG P41 Ab, P41 Ab IgG, or Bb flagellin Ab IgG) is one of 10 IgG bands on the Lyme disease Western blot
• A Reactive, Present, or Abnormal result on this band alone is not a positive Lyme test — 5 of 10 IgG bands must be reactive to meet CDC surveillance criteria
• The P41 antigen is a flagellin protein shared by many bacteria — it is the most cross-reactive and least Lyme-specific band on the entire panel
• False-positive 41 kD IgG results are common in lupus, rheumatoid arthritis, Epstein-Barr virus, syphilis, and in people with no Lyme exposure at all
• If you see "IgG P41 Ab: Present" or "41 kD IgG: Reactive" on your report, the key question is how many other IgG bands are reactive alongside it
• IgG antibodies persist for years — a reactive result may reflect past treated exposure, not active disease

What Is the 41 kD IgG Band?

The Western blot (immunoblot) is the second step in the CDC two-tier algorithm for Lyme disease testing. It identifies which Borrelia burgdorferi proteins your immune system has produced antibodies against, separated by molecular weight in kilodaltons (kD).

The 41 kD band targets the flagellin protein of Borrelia burgdorferi — the structural protein forming the bacterial flagellum. It is also called P41, IgG P41 Ab, Bb flagellin Ab IgG, or flagellin IgG depending on the laboratory.

How it appears on your report:

• 41 kD IgG: Reactive / 41 kD IgG: Non-reactive
• IgG P41 Ab: Present / IgG P41 Ab. Present Abnormal
• WarningAbnormal IgG P41 Ab / IgG P41 Ab. 01 Present
• P41 Ab IgG: Reactive / Bb Flagellin Ab, IgG
• Flagellin (P41) z Borrelia afzelii: Positive (European labs)
• Antygen P41: Dodatni (Polish) / Borrelien P41 positiv (German)
• Protéine P41 positif / Anticorps P41 positif (French)

Does a Reactive 41 kD IgG Band Mean I Have Lyme Disease?

Reactive or Present means IgG antibodies in your blood bound to the 41 kD flagellin protein during the Western blot procedure. Non-reactive, Absent, or Negative means no binding occurred.

The word Abnormal on a Western blot band result means the band was reactive when the expected (normal) result is non-reactive. It does not mean you have Lyme disease — it means this one band showed antibody activity outside the expected negative result.

This is not a positive Lyme test. The 41 kD band is the most cross-reactive and least diagnostically specific band on the IgG Western blot panel because flagellin is not unique to Borrelia burgdorferi. Studies of healthy urban populations with no Lyme disease history have found reactive 41 kD bands in a significant proportion of people. The likely reason: non-pathogenic spirochete bacteria in the human mouth and gut share the same flagellin protein and can trigger antibody cross-reactions on this band.

Is My Lyme Test Positive If Only the 41 kD Band Is Reactive?

No. CDC surveillance criteria define a positive IgG Western blot as 5 or more of the following 10 bands reactive:

The 39 kD (BmpA) and 21 kD (OspC) bands carry the most diagnostic weight when present. The 41 kD band is at the opposite end — reactive alone, it adds almost nothing to a Lyme diagnosis.

Note: these are CDC surveillance criteria used to guide standardized testing. Clinical Lyme disease diagnosis requires interpretation alongside symptoms, tick exposure history, and clinical evaluation — positive serology alone does not necessarily indicate active infection requiring treatment, and negative serology does not exclude early Lyme disease when symptoms are consistent.

Interpretation Flowchart

41 kD IgG Band Reactive and Lupus: What You Need to Know

Why lupus causes false-positive 41 kD IgG results:

Lupus involves chronic, widespread immune activation — the immune system produces antibodies against many different antigens simultaneously (polyclonal activation). This includes antinuclear antibodies (ANA), which are the hallmark of lupus, but also non-specific antibodies that can cross-react on bacterial protein assays including the Lyme Western blot. The 41 kD flagellin band is particularly vulnerable to this cross-reactivity because flagellin is structurally common across many bacterial species.

Several studies have documented false-positive Lyme Western blot results in patients with lupus, including isolated reactive bands — most commonly the 41 kD band.

Symptom overlap between lupus and Lyme disease:

What a reactive 41 kD IgG means if you have lupus:

A reactive 41 kD IgG band in a person with known or suspected lupus is more likely to reflect lupus-associated false positivity than Lyme disease exposure, particularly if fewer than 5 IgG bands are reactive, you have no significant tick exposure history, and your symptoms are better explained by lupus activity than Lyme disease.

This does not mean co-occurrence is impossible — you can have both conditions. But an isolated false-positive 41 kD band with fewer than 5 total reactive IgG bands is not evidence of Lyme disease.

What to do: discuss the result with your rheumatologist in the context of your lupus workup. A reactive 41 kD band alone should not change lupus management or prompt Lyme treatment without full clinical evaluation and, if warranted, a repeat two-tier Lyme test.

What If the 41 kD IgG Band Is "Abnormal" or "WarningAbnormal"?

These portal labels are simply flags indicating the band result was reactive when the expected result is non-reactive. They are automated portal flags — not clinical diagnoses. "Abnormal" here means "different from the expected negative" — it does not mean you have Lyme disease.

Some patient portal systems display this result with alarming language — "WarningAbnormal IgG P41 Ab" — that can cause significant anxiety. The clinical significance is exactly the same as "Reactive": one band positive on a 10-band panel that requires 5 positive bands to be considered a positive test.

What If I Have the 41 kD Band Plus Other Bands Reactive?

If the 41 kD band is reactive alongside other bands, the clinical picture changes depending on which bands and how many:

41 kD + 58 kD reactive (2 bands): Still negative by CDC criteria. The 58 kD band is moderately specific for Lyme; having both reactive raises suspicion more than the 41 kD alone, but remains below the 5-band threshold.

41 kD + 39 kD reactive (2 bands): Still negative, but the 39 kD BmpA band is highly Lyme-specific. This combination with consistent symptoms and tick exposure warrants careful clinical assessment and possibly repeat testing in 2–4 weeks.

41 kD + 58 kD + others = 5+ bands: Meets CDC positive criteria. Interpret alongside first-tier ELISA result, symptoms, and exposure history.

23 kD IgM + 41 kD IgG reactive simultaneously: These are on different panels. The 23 kD IgM band is one of the three IgM bands — if 2 of the 3 IgM bands are reactive, that is a separate positive IgM Western blot result, evaluated independently of the IgG panel.