Understanding the disease

Meet the IgG family and learn why IgG4, usually a peacekeeper, appears in disease inflammation.

Understanding the IgG antibody family and the special role of IgG4

Before we can understand why IgG4-related disease behaves the way it does, we first need to understand the molecule at the center of its story: IgG4. Think of your immune system as a well-trained team of protectors. Among them, antibodies—also called immunoglobulins—are the scouts. They look for trouble, mark threats, and help keep you healthy.

IgG is the most common family of antibodies in the body. But within that family, there are four siblings—IgG1, IgG2, IgG3, and IgG4—each with a unique personality. IgG4 is the quiet, gentle sibling. It usually shows up in situations where the immune system wants to calm things down, not ramp things up. That’s part of what makes IgG4-related disease so surprising: a molecule known for not making a fuss is involved in an illness marked by inflammation and fibrosis.

In this lesson, we’ll break down what IgG molecules do, why IgG4 is different, and how those differences help explain the disease.

What is IgG?

IgG: A common antibody in everyday immune defense

IgG stands for immunoglobulin G, one of the most abundant types of antibodies in your bloodstream. By binding to viruses, bacteria, and toxins, it marks them for removal by the immune system. These antibodies:

• Circulate in the blood and tissues.

• Recognize specific targets called antigens, the unique “fingerprints” of microbes or other molecules.

• Recruit other immune cells to neutralize and remove what they bind.

Medical research indicates that antibodies IgG1, IgG2, and IgG3 often stimulate stronger inflammatory signals, including activation of the complement system (a protein cascade that boosts inflammation).

IgG4, however, behaves differently.

What makes IgG4 unique?

IgG4: Distinct features

IgG4 has several features that set it apart:

• It generally does not strongly activate the complement system, which is the protein cascade that boosts inflammation.

• It binds less tightly to several Fc receptors, so it sends fewer activating signals to some immune cells.

• It often increases with repeated or long-term exposure to the same antigen (for example, some allergies or chronic exposures).

• It is often postulated to play a role in the prevention of serious allergic responses, such as anaphylaxis resulting from bee stings.

• It can undergo Fab-arm exchange—two IgG4 antibodies swap halves—creating bispecific antibodies (two different arms).

What is Fab-arm exchange?

Fab is the acronym for fragment antigen-binding—the part of an antibody that grabs a specific target (antigen).

Because IgG4 can swap half of itself with another IgG4 molecule, it often ends up with two different “arms.” Each arm recognizes a different target (making IgG4 an “asymmetric” antibody), so it’s less effective at cross-linking cells and is less likely to set off strong inflammatory cascades.

This Fab-arm exchange is what researchers describe¹ as one of IgG4’s defining features. It makes IgG4 less likely to “clump” and activate strong immune reactions.

Why does this matter?

IgG4 typically acts as a buffer, reducing allergic or inflammatory responses. It’s the molecule your immune system uses when it decides:

“Let’s keep things from getting too out of hand.”

Which is why IgG4-related disease is so surprising: the very molecule associated with “calming the immune system” is also linked to an illness marked by inflammation, swelling, and scarring.

How does IgG4 relate to IgG4-related disease?

Current research^2,3,4 suggests that:

• Rising IgG4 levels are often a marker of broader immune pathway activation rather than the direct cause of tissue injury.

• Elevated IgG4 concentrations in the blood may represent a “counter-regulatory” effort by the body to suppress the primary source of ongoing inflammation.

• Immune cells within the patient’s own immune system, such as T follicular helper (Tfh) cells, regulatory T cells (Tregs), plasmablasts, and fibroblasts, play important roles.

• These pathways are associated with fibrosis (scarring) and lymphoplasmacytic inflammation seen in IgG4-RD.

In short, IgG4 can be elevated in the blood or tissues, but the molecule does not appear to be the main driver of organ damage. Instead, it points to immune pathways that need attention.¹

Summary

IgG antibodies help your immune system recognize and clear threats. IgG4, one of the four IgG subclasses, has distinct features, including undergoing Fab-arm exchange and producing bispecific antibodies. It typically doesn’t activate inflammation and often works to quiet the immune response.

In IgG4-RD, IgG4 often serves as a marker of immune activity while cells like B cells and T cells, along with fibroinflammatory signals, contribute to the disease process. Understanding IgG4 helps explain why this condition can be hard to identify and how treatments that affect B cells may help reduce disease activity.

References

1. Stone JH, Zen Y, Deshpande V. IgG4-Related Disease. N Engl J Med. 2012;366(6):539-551. URL: https://www.nejm.org/doi/10.1056/NEJMra1104650

2. Arias-Intriago M, Gomolin T, Jaramillo F, et al. IgG4-Related Disease: Current and Future Insights into Pathological Diagnosis. Int J Mol Sci. 2025;26(11):5325. DOI: https://doi.org/10.3390/ijms26115325

3. Akiyama M, Alshehri W, Ishigaki S, Saito K, Kaneko Y. The immunological pathogenesis of IgG4-related disease categorized by clinical characteristics. Immunological Medicine. 2025;48(1):11-23. DOI: https://doi.org/10.1080/25785826.2024.2407224

4. Peyronel F, Della-Torre E, Maritati F, et al. IgG4-related disease and other fibro-inflammatory conditions. Nat Rev Rheumatol. 2025;21:275-290. DOI: https://doi.org/10.1038/s41584-025-01240-x

5. Lang D, Zwerina J, Pieringer H. IgG4-related disease: current challenges and future prospects. Ther Clin Risk Manag. 2016;12:189-199. DOI: http://dx.doi.org/10.2147/TCRM.S99985