Antibodies are the fundamental component of humoral immune responses, also known as antibody-dependent responses. These molecules can recognise and neutralise pathogens, either by binding to molecular antigens and directly preventing their pathogenic effect, or by opsonising these pathogens and triggering effector functions, such as the complement system, antibody-dependent cell-mediated cytotoxicity and antibody-dependent cellular phagocytosis.

Five different classes of antibodies, or immunoglobulins (Ig) are found in humans, namely IgM, IgD, IgA, IgE and IgG. IgM and IgD are involved in primary adaptive humoral immunity and can be found in mature naïve B cells (1), which undergo specialisation and a class switch towards long lived B cells and plasmablasts that produce IgA, IgE and IgG (2, 3). IgA is mainly involved in mucosal humoral immunity. IgE is one of the main components of allergic reactions. IgG is part of several different processes both in health and disease.

IgG has four subclasses (i.e., IgG1, IgG2, IgG3 and IgG4) which form part of the immune response. Under normal conditions, IgG1-3 can fixate complement and opsonise pathogens, whilst IgG4 only activates complement under special circumstances, at high antibody and antigen concentrations. IgG and its subclasses are involved in autoimmune processes, such as rheumatoid arthritis, systemic lupus erythematous, ANCA-associated vasculitides, as well as IgG4-autoimmune diseases (IgG4-AID) and IgG4-related disease (IgG4-RD).

In this review, we describe the structure and function of IgG4 antibodies, discuss the role of this class in allergy, helminth infections, malignancies, and autoimmune diseases. A greater emphasis is placed on IgG4-related disease and the role of IgG4 antibodies in the pathogenesis of the disease.

Immunoglobulin G has four subclasses, with IgG4 representing up to 5% of the total IgG concentration (4). Their structure is based on 2 heavy and 2 light chains bound together by disulphide bridges. There are interactions between the light and heavy chains, as well as a connection between the two heavy chains at the hinge region. This region is important because it gives structural flexibility to the molecule. Both the light and heavy chains have antigen-binding sites, the variable region (V_L and V_H), and areas responsible for the effector function of the antibodies, called constant region (C_L and C_H1, C_H2, C_H3, C_H4). Furthermore, these molecules can be divided by function, the “Fragment, antigen binding” (Fab) region and the “Fragment, crystallised” (Fc) region, which is responsible for the effector function (5). In the CH2 domain, a switch of proline to serine at position 228 in the hinge of IgG4 facilitates Fab-arm exchange ( Figure 1 ). Schuurman, Aalberse and colleagues observed that this phenomenon is marked by the dissociation of the two heavy chains and recombination of two random IgG4 monomers (heavy and light chains) to form a bispecific dimer (6). Even though this new molecule might recognise two different antigens, it is functionally monovalent, and cannot form large immune complexes (7–9). The Fc portion in IgG4 antibodies have a high affinity for antigens and FcγIIb receptors but low affinity for Fcγ stimulatory receptors and cannot tipically activate the classical complement pathway (10). These characteristics hinder Fc mediated effector functions and prevent further sensitisation of the immune system (8, 11–13). There are, however, situations where IgG4 molecules can activate the complement pathway. The first is via recruiting mannose-binding lectins and activating the lectin pathway, which has been observed in anti-PLA2R membranous nephropathy (14). The second is through mutations of the Fc region, enabling these molecules to form hexamers and bind to C1q (15). These molecules can also activate the complement system using the classical pathway when there is high antigen density and high antibody concentration (16). Finally, Oskam, Rispens and collaborators showed that in opposition to previous evidence, the heavy chains of IgG4 molecules can dissociate and interact with the Fc region of other IgGs. It is not clear whether this mechanism plays a role in the pathogenesis of autoimmune diseases, but it modulates the affinity of IgG1 and IgG2 for C1q (17). Hence, IgG4 has anti-inflammatory properties, and it is involved both in health, such as the development of tolerance in allergies (18), as well as disease, such as curbing immune responses against helminth infections (19) ( Table 1 ).

When exposed to antigens, naïve B cells become antibody secreting B cells via the extrafollicular and follicular pathways, which are complementary. The initial response to antigens is predominantly driven by the extrafollicular path (20, 21). It involves T follicular helper (Tfh) cells driving the differentiation of naïve B cells into short lived plasmablasts, which will maintain an immune response for up to a week after the trigger (20, 22).

To create an immune response based on IgG4, B cells must undergo class switch recombination (CSR) and somatic hypermutation (SHM) (11, 23, 24). These processes are hallmarks of follicular responses, which take part in germinal centres (GC) of lymph nodes and the spleen. In the follicular pathway, antibody secreting B cells develop high affinity and specificity and become either memory B cells or long-lived plasma cells (LLPC). The GCs are divided in dark and light zones, with functional differences. The dark zone promotes proliferation of B cells, as well as CSR and SHM (25). CSR is marked by deletional recombination in the heavy chain gene that leads to a change in Ig class, thus changing effector functions of the antibodies, while preserving the affinity for specific antigens. Given its mechanism, switch in Ig class follows a sequential pattern from IgM/IgD to IgG3, IgG1 and IgA1, called proximal classes. A secondary switch then gives rise to distal classes such as IgG2, IgG4 and IgA2 (23, 26, 27) and has been attributed to stimuli from T helper 2 (Th2) cells. Interleukin (IL) 4 and IL-13 are responsible for skewing the antibody production to IgG4 and stimuli involving IL-10 and/or IL-21 lead to selection of IgG1 antibodies (28). After these clones have developed high-affinity and specificity, they migrate into the light zone. There, they will undergo a selection according to interactions with Tfh cells and follicular dendritic cells (29). Once the highly specific clones are selected, they leave the GC and become LLPC or migrate to the bone marrow as memory B cells.

IgG4 antibodies have an important role in the development of tolerance in atopic patients. It has been extensively shown that beekeepers develop an IgG4-based response to bee venom associated with reduction of symptoms after exposure to the toxin (30, 31), and the same event has been described in other cases of occupational exposure (32, 33). Furthermore, allergen-specific immunotherapy is based on the development of IgG4 antibodies against allergens through regular and incremental exposure to allergenic antigens (34). IgG4 competitively binds to allergens, thus hindering the formation of IgE-antigen immune complexes (35, 36). Furthermore, it also binds to FcγRIIb, an inhibitory Fcγ receptor, preventing the degranulation of mast cells and the cascade that would lead to allergic responses (37–39).

On the other hand, IgG4 antibodies are also involved in the maintenance of the disease in cases of eosinophilic oesophagitis (EoE) and eosinophilic chronic rhinosinusitis (40–42). The most robust evidence that this class is involved in disease comes from studies associating it with symptoms in patients with EoE. During food avoidance tests, patients referred less symptoms and biopsies showed reduction of IgG4 deposition (43). In addition, histological levels of IgG4 were found to be significantly lower during remission when compared to samples collected during disease activity (44). For a detailed review on the topic, please see (45).

Helminthic infections are another scenario where IgG4 antibodies show their tolerogenic effects. Patients with acute parasitic infection usually show a balanced Th1/Th2 immune response, with a predominance of IFN-γ over IL-10, and a higher count of peripheral eosinophils (46). If the pathogen fails to evade the host response, this leads to parasite clearance and termination of the infection.

Individuals with chronic infection, however, show an immune response with a Th2 profile, where IL-10 plays a critical modulatory role in skewing antibody production towards IgG4 (46). A response based on parasite-specific IgG4 antibodies, then, dampens IgE-mediated immune response and perpetuates the infection. Asymptomatic patients show a restricted response to parasite antigens, while those with severe chronic disease have lower levels of serum IgG4 antibodies and show signs of organ damage (46). Studies also show that individuals with a high parasite burden have higher levels of parasite-specific IgG4 than patients with a lower parasite burden (47, 48). The immunomodulatory effects of helminthic infections are also seen in the response to allergens, where patients with chronic parasitic infection show lower levels of IgE-reactivity to dust antigens (49).

Antitumour humoral responses can prevent malignancies from developing further and even annihilate them by triggering humoral and cell-mediated immune responses. Some neoplasms induce class switch of anti-tumour antibodies to the IgG4 class, leading to immune escape and IgG4-mediated tumour-tolerogenic responses (50). Melanoma and pancreatic ductal adenocarcinoma cells induce a modified Th2 immune response around their microenvironment, triggering B cells to undergo antibody class switch towards IgG4 (51, 52). These antibodies will then competitively bind to cancer antigens and damp anti-tumour immune responses. Higher serum levels of IgG4 are associated with fewer cytotoxic T cells (53) and with a worse prognosis in patients with cholangiocarcinoma and melanoma (50, 54).

A meta-analysis reported that patients with IgG4-RD have an increased risk of clonal B cell lymphoma and pancreatic malignancy when compared with a matched general population (55). The mechanisms driving this increased risk remain speculative and include 1) chronic inflammation-induced carcinogenesis; 2) IgG4-related tumour-induced immune escape; 3) paraneoplastic phenomenon, as most malignancies detected distant from the site of disease activity. The class switch towards IgG4 induced by some malignancies may also play a role in the initiation of IgG4-RD. K-ras codon gene mutations associated with gastrointestinal cancer have been reported in patients with IgG4-related pancreatitis (56). These mutations are also associated with cellular transformation and genetic instability (57, 58).

Huijbers et al. were the first to collectively describe IgG4 autoimmune diseases (IgG4-AID) as a spectrum of conditions characterised by autoantibody responses against a known antigen (59). Evidence shows that these diseases are caused by IgG4 antibodies recognising autoantigens, although not all antigens have been validated. The diagnosis of IgG4-AID depends on high level of suspicion and identification of IgG4 autoantibodies against the antigen(s) specific to each disease (60). Koneczny proposed a classification system based on evidence of antibodies against extracellular antigens, and direct pathogenic mechanism of IgG4 antibodies observed in vitro and validated in animal models through passive transfer (61). Although 29 antigens have been described in IgG4-AID ( Table 2 ), only the six antigens have met all three Koneczny criteria of IgG4 pathogenicity (61). For a thorough review on IgG4-AID and autoantigens involved in these disorders, please read (62).

IgG4-related disease (IgG4-RD) is a chronic relapsing immune-mediated fibro-inflammatory disorder and its hallmark is the presence of IgG4 antibodies in the sites affected (89). The diverse organ involvement in this disease is linked by a unique histopathology; a lymphoplasmacytic infiltrate rich in IgG4+ plasma blasts and CD4⁺ T cells, storiform fibrosis, and obliterative phlebitis (90).

There are currently three theories on the role of IgG4 antibodies in IgG4-RD: 1) IgG4 antibodies directly cause the disease by targeting self-antigens; 2) IgG4-RD patients have an immune response inherently biased towards production of IgG4 antibodies to any stimulus; 3) IgG4 is only present to modulate an immune response based on different pathways. It continues to be elusive whether the overexpression of these antibodies is an epiphenomenon of the inflammatory reaction or has a causal role in disease pathology.

The antigens proposed for IgG4-RD are ubiquitous and found both in the cytosol and the extra cellular matrix. Nevertheless, with the exception of laminin 511-E8, they fall short of meeting either Witebsky or Koneczny criteria for autoimmunity (61, 146). This brings into question whether these antibodies are the drivers of IgG4-RD. There is evidence that antibodies can penetrate the plasma membrane and recognise intracellular antigens (147–149). In fact, many entry mechanisms have been studied, such as interaction of basic residues with a negatively charged cell membrane (150), Fc-receptor mediated entry (151), endocytosis (147, 152, 153), and nucleoside transporters (154). Among the six IgG4-AID that meet IgG4 autoimmunity criteria, however, all the antigens involved in their pathogenesis are found in the cell membrane or free in the blood circulation (62).

Another important consideration when analysing the role of autoantigens in IgG4-RD is the clinical presentation of the disease. Regardless of phenotype (i.e., pancreatobiliary, head and neck, retroperitoneum and aorta, and systemic), the hallmarks of IgG4-RD are proliferative and/or fibrotic lesions in the organs affected. These findings are diametrically opposed to those from IgG4-AID, where symptoms are driven by IgG4 antibodies blocking antigen function, such as in CNTN1-associated CIDP (155). Histological examination of IgG4-RD lesions shows a massive invasion of IgG4⁺ B cells and plasmablasts and extensive (storiform) fibrosis (156). Furthermore, CD4⁺ and CD8⁺ cytotoxic T lymphocytes are also part of the immune response found in organs with active inflammation and peripheral blood (157), suggesting that the interaction between T and B cells play a critical role in the damage caused by IgG4-RD.

Despite recent developments in IgG4-RD research, there are essential questions without an answer. One of the most important ones is why and how patients develop it. Previous studies reported an association of HLA-DRB1 and FCGR2B with IgG4-RD, including with phenotypical characteristics (98), and two studies observed a relation between HLA genes, particularly HLA-DQB1*04:01", HLA-DRB1*04:05 and HLA-DRB1*16 in patients with autoimmune pancreatitis type 1 and 2 (158, 159). Moreover, it is also important to look at B cell specialisation and how it can impact humoral immune responses favouring IgG4 expression. As in IgG4-AID, it is paramount to identify whether naturally occurring antigen(s) is(are) driving the immune response in IgG4-RD and the evidence surrounding self-antigens needs to be validated in external cohorts. We must consider the possibility that, such as IgG4-AID, IgG4-RD is a spectrum of diseases rather than a single entity. Finally, the increased expression of IgG4 antibodies and the presence of IgG4⁺ plasma cells in sites with IgG4-RD inflammation highlights the importance of clarifying the mechanisms that favour class switch towards IgG4 rather than other IgG subclasses. This knowledge is crucial to understand the pathogenesis of IgG4-RD (160).

RVM: Conceptualization, Data curation, Writing – original draft, Writing – review & editing. EC: Conceptualization, Data curation, Supervision, Writing – original draft, Writing – review & editing.