The major histocompatibility complex (MHC) is a highly polymorphic region located on the short arm of chromosome 6 (6p21) and is an essential component of the immune response. The MHC is considered the region most associated with human diseases in the genome. The MHC genomic region is also known as the HLA region in humans. HLA antigens are divided into three classes, namely, HLA class I (HLA-A, -B, and -C), HLA class II (HLA-DR, -DQ, and -DP), and HLA class III (including complement and cytokine genes) (19, 20). HLA-G is a non-classical HLA class I molecule that regulates the balance between Th1 and Th2 cells (21). Recent studies have suggested that HLA class I genes may be closely related to autoimmune diseases, as they are involved in processing and presenting peptides for T cell recognition and are also associated with transplant compatibility. Fang et al. (22) were the first to report that the HLA-A11:01, -B37:01, and HLA-G*0101, 0106 alleles are associated with BP susceptibility in the northern Han Chinese population. Chagury et al. (23) reported that the HLA-C*17 allele is associated with the development of BP in the Brazilian population. However, Christian et al. (24) tested the HLA-Cw6 allele in 40 pemphigus vulgaris patients and 40 BP patients; they detected the HLA-Cw6 polymorphism in 4 out of 40 BP patients, and they reported an HLA-Cw6 genotype frequency of 10% in the BP group but without statistically significant differences between the groups. Banfield et al. (25) and Schaller et al. (26) also reported no significant association between HLA class I genes and BP in the British Caucasian population. The conflicting conclusions may stem from variations in geographic location and population differences in race and ethnicity linked to genetic risk factors. Additionally, confounding variables, such as socioeconomic status, access to healthcare, and the impact of specific environmental factors, may significantly contribute to the various conclusions (27, 28). Future studies are essential to further elucidate the association between HLA class I genes and BP.

Most studies have reported the widespread presence of HLA lass II genes in BP patients, with susceptibility showing ethnic differences. Seignalet et al. (29) were the first to report the relationship between HLA and BP in French patients, identifying a potential association between HLA-DR5 and BP. Subsequently, Okazaki et al. (30) reported an association of the HLA-DRB1*04 (0403, 0406) and DRB1*1101 alleles with BP susceptibility in Japanese patients. However, a small-sample study conducted in the Chinese population has demonstrated that the HLA-DRB108 allele has a protective effect against BP. Recently, Fang et al. (22) also reported that in the Han Chinese population, the HLA-DRB1*07:01 allele has a protective effect against BP, while the HLA DRB1*10:01 allele is associated with BP susceptibility. The first genome-wide association study (GWAS) conducted in Germany has revealed that the HLA-DRB1*07:01 and HLA-DQA1*05:05 loci have the strongest association with BP (31). Andreani et al. (32) analyzed 30 cases of idiopathic BP and 86 cases of dipeptidyl peptidase-4 inhibitor (DPP4i, such as gliptin drugs)-related BP, and they identified a significant association of the Italian BP population with the DRB1*11:01, DRB1*11:04, and DRB3*02:02 alleles.

Neutrophils are the frontline responders in the acute innate immune response to invading pathogens and are the most abundant effector cells in the human immune system. Neutrophils play a crucial role in maintaining homeostasis and supporting immune responses within the body (103). In BP, neutrophils are one of the first inflammatory cell types to infiltrate skin lesions and are key cellular elements in triggering dermal-epidermal separation (104). Neutrophils are critical effector cells in the pathogenesis of blister formation in experimental BP models. Subepidermal blistering in these models is mediated by the interaction between the Fc region of anti-BP180 IgG and the Fc receptors on neutrophils. As previously mentioned, a high copy number of FcgRIIc and a low copy number of FcgRIIIb may be associated with increased susceptibility to BP (53). FcgRIII, in particular, is a key receptor that facilitates the binding of pathogenic anti-mBP180 IgG and activates infiltrating neutrophils (105). This process is also dependent on the activity of NE (104). NE is an enzyme secreted by the first wave of activated neutrophils that cleaves mBP180 within the immunodominant NC14A domain, generating a 12-kD degradation product known as p561, which exhibits chemotactic effects on neutrophils, both in vitro and in vivo ( 106). NE activity is crucial for the formation of subepidermal blisters in experimental BP. NE acts alone or in combination with other effector molecules to degrade components of the BMZ that maintain dermal-epidermal cohesion, thereby playing a pathogenic role in BP (107). In addition to NE, neutrophil granules contain several proteolytic enzymes, including cathepsin G (CG), collagenase, and gelatinase B (GB). GB released by neutrophils plays a key role in subepidermal blister formation in experimental BP. GB can directly cause tissue damage in BP by cleaving structural proteins at the dermal-epidermal junction or indirectly contribute to tissue damage by inactivating major neutrophil elastase inhibitors, such as α1-PI, or other neutrophil-derived protease inhibitors. These enzymes may also play a role in the long-term progression of the disease (108). As previously mentioned, CXCL10 produced by macrophages stimulates neutrophils to secrete MMP-9, thereby potentially driving an inflammatory loop that is associated with disease outcomes in BP patients (90). In contrast, neutrophil-produced ROS and neutrophil-derived nicotinamide adenine dinucleotide phosphate (NADPH) oxidase have been shown to play a crucial role in tissue damage induced by autoantibodies in a BP cryosection model (109). ROS and reactive nitrogen species, key redox molecules in immunity, are produced by activated neutrophils and monocytes through NADPH oxidase and NOS. Neutrophils produce higher levels of ROS, while monocytes generate more reactive nitrogen species (110). Monocytes also promote neutrophil recruitment and ROS production, leading to significantly increased dermal-epidermal separation (111). Additionally, we previously reported that IL-8 (rs4073) affects BP susceptibility. Neutrophils release neutrophil extracellular traps (NETs) through apoptosis. Studies have shown that the number of NETs in the skin lesions, serum, and blister fluid of BP patients is higher than in corresponding samples from healthy controls. Elevated levels of key inflammatory factors, such as IL-8, in BP serum and blister fluid induce NET formation. The BP immune microenvironment contains immune complexes that bind to Fc receptors on neutrophils, inducing NET formation in circulation and skin lesions. These abnormal NETs promote B cell differentiation into plasma cells and enhance the production of autoantibodies by activating the MAPK p38 cascade (112). In addition, the IL-17A and IL-23 cytokines are elevated in the serum of BP patients at risk of disease recurrence. The IL-17/IL-23 axis promotes various pathological processes, including stimulating the production of neutrophil MMP-9 and NE, as well as the release of NETs ( Figure 2 ) (97, 113). After IL-17 recruitment and activation, neutrophils produce more IL-17 and release proteases, leading to matrix degradation and the generation of matrix peptides, which further enhance neutrophil recruitment and activation at the site of skin lesions (88). Recent studies have revealed a strong correlation between significant neutrophil infiltration and comorbid psoriasis in BP patients (114). In an ex vivo model using normal human skin cryosections, BP patient IgG-induced separation of the dermal-epidermal junction (DEJ) depends on neutrophils activated by immune complexes (115). In summary, neutrophils play a pivotal role in the pathogenesis of BP through multiple mechanisms, including Fc receptor-mediated antibody interactions, release of proteolytic enzymes, generation of oxidative stress, and activation of cytokine networks. Future studies should further elucidate the precise mechanisms of neutrophil involvement in BP, which may provide novel therapeutic targets for this disease.

Eosinophil infiltration and peripheral eosinophilia are considered early and key events in the development of BP lesions. Peripheral eosinophilia is observed in 50% to 60% of BP cases and is positively correlated with disease severity (116, 117). Of note, healthy relatives with high-risk HLA class II alleles exhibit T cell responses after treatment with recombinant NC16A. In individuals with BP, a Th2 response is observed, and eosinophils drive Th2 polarization through differential cytokine release (118). As previously mentioned, BP is associated with IL-13 gene polymorphisms. Hashimoto et al. (119) suggested that IL-13 contributes to BP-related pruritus by directly stimulating peripheral nerve fibers and/or indirectly by recruiting eosinophils, which then promote peripheral nerve damage. Compared to patients with normal eosinophil counts and percentages, BP patients with serum eosinophilia tend to be older, with more severe palmar-plantar involvement and a higher proportion of indirect immunofluorescence positivity (120). De Graauw et al. (121) demonstrated that in the presence of BP autoantibodies, IL-5-activated eosinophils cleave the skin at the DEJ, directly causing subepidermal blister formation in BP. In skin lesions, eosinophils are activated to release the major basic protein, eosinophil-derived neurotoxin (EDN), and eosinophil cationic protein (ECP). High concentrations of ECP and EDN are present in the serum and blister fluid of BP patients (122). Intradermal injections of EDN and ECP into guinea pig skin over six weeks leads to ulcerative or crusted lesions accompanied by marked cellular infiltration, indicating that these proteins actively contribute to BP skin pathology (123). Furthermore, Amber et al. (124) suggested that ECP and EDN may promote BP pathogenesis by directly affecting keratinocytes, inducing the expression of BP-related cytokines, chemokines, and MMP-9, as well as impairing cell viability and extracellular matrix adhesion. Tsuda et al. (125) also demonstrated that eosinophil degranulation directly damages basal keratinocytes, leading to separation at the DEJ. Eosinophils express receptors for the C3a and C5a complement anaphylatoxins, which not only regulate eosinophil migration but also trigger degranulation. Previous studies have suggested that complement activation plays a critical role in BP blister formation (126). Eosinophils are also key contributors to the secretion of MMP-9 (127). IL-3 and TNF-α stimulate eosinophils to produce large amounts of MMP-9 (128). In a mouse model, MMP-9 has been reported to regulate NE activity by inactivating α1-proteinase inhibitor, leading to further degradation of BP180 and separation of the DEJ (107). Eosinophils are also an important source of tissue factor (TF) in the vasculature, serving as initiators of the extrinsic coagulation pathway. Eosinophils play a role in coagulation activation through TF (129). Tedeschi et al. reported that the levels of ECP in the blister fluid of BP patients are significantly elevated and correlated with coagulation activation markers, leading to inflammation, tissue damage, blister formation, and a potential risk of thrombosis (130, 131). Additionally, ex vivo experiments using human skin and isolated human eosinophils have indicated that eosinophil extracellular traps (EETs) may contribute to DEJ separation, as DEJ separation is significantly reduced after DNase treatment (132). These extracellular traps have been identified in human biopsy skin samples from a range of diseases, including BP (127). Eosinophils also participate in the pathogenesis of BP by mediating the effects of anti-BP180 IgE antibodies and promoting dermal-epidermal separation. When anti-NC16A IgE is introduced in vivo, eosinophils are essential for inducing BMZ separation (127, 133). Conversely, Gounni Abdelilah et al. (134) reported that the levels of eosinophil chemotactic factors and MCP-4 are elevated in tissues and blister fluid of BP patients and may be secreted by eosinophils upon stimulation by IgG, IgA, or IgE immune complexes induced by IL-5. An important autocrine pathway may be involved in the recruitment and activation of local eosinophils in BP. In addition to CCL11, Günther et al. (135) linked the upregulation of the eosinophil chemotactic factor CCL26 in BP to the accumulation of activated eosinophils in skin lesions. This finding expands the understanding of BP pathogenesis and may suggest new options for therapeutic intervention. Eosinophils are the major source of IL-31 in BP, and this cytokine may contribute to pruritus in BP patients (136). Importantly, eosinophils from BP patients release higher levels of IL-31 compared to those from healthy donors ( Figure 2 ) (137). Elevated serum IL-31 levels in BP patients have also been shown to correlate with peripheral eosinophil counts and the presence of anti-BP180 IgE antibodies. Substance P induces eosinophils to release nerve growth factor (NGF) and IL-31. Due to its ability to sensitize primary pruriceptive neurons, NGF may play an important role in mediating itching (138). In summary, eosinophils play a pivotal role in BP pathogenesis through direct tissue damage and immunomodulatory functions. Their activation involves multiple interconnected pathways, including cytokine networks, the complement system, chemokine axes, and neuro-immune crosstalk. While therapeutic strategies targeting these pathways hold significant clinical potential, further research is needed to elucidate eosinophil heterogeneity and optimize targeting approaches.

Basophil infiltration has been observed in various inflammatory skin diseases, including atopic dermatitis, prurigo, and urticaria. This phenomenon has also been observed in autoimmune skin diseases, such as BP (139). However, few studies have investigated the role of basophils in the pathogenesis of BP. In 1982, Dvorak et al. (140) used electron and light microscopy to study the inflammatory response in lesions at different stages of clinical development in BP patients; these researchers were the first to report basophil infiltration in BP skin lesions, primarily found in clinically significant lesions and adjacent normal skin. Moreover, the location of eosinophils within BP lesions is related to their proximity to degranulating basophils. Recently, Hashimoto et al. (119) analyzed skin lesions from 24 BP patients using immunofluorescence staining and reported that the number of basophils infiltrating the dermis is significantly higher in BP skin samples compared to healthy controls. In addition, the number of dermal basophils is significantly correlated with the severity of itching, highlighting the relationship between basophils and BP-associated pruritus. Basophils produce IL-31, which leads to severe itching (141). High levels of IL-31 have been detected in the serum and blister fluid of BP patients. Basophil-secreted IL-31 recruits eosinophils to the lesion site. When recruited eosinophils produce IL-4 and IL-13, more basophils are activated to produce IL-31, thereby forming a positive feedback loop that promotes BP-related pruritus ( Figure 2 ) (142). Kimura et al. (143) compared erythematous and bullous lesions in a total of 25 BP patients using histopathology, immunohistochemistry, and electron microscopy, confirming the dual role of basophils in the development and resolution of BP; they reported a positive correlation between the number of basophils and eosinophils in the early stages of BP. During the bullous phase, the number of basophils is significantly higher, but in the later stages, basophils interact with M2 macrophages to inhibit disease progression.

As previously discussed, IL-1β plays a crucial role in BP by inducing the secretion of MMP-9 in macrophages and promoting neutrophil recruitment, degranulation, and NET formation (145). The IL-8 and IL-17/IL-23 axis also influences BP susceptibility and triggers NET formation. Therefore, targeting the IL-1β pathway, particularly the NLRP3 inflammasome, in combination with inhibiting IL-8, the IL-17/IL-23 axis, NE, and MMP, may be a promising approach for BP patients. AC-203 is a topical formulation that modulates the inflammasome and IL-1β pathways. A Phase 2 open-label clinical trial (NCT03286582) has compared 1% AC-203 ointment with 0.05% clobetasol ointment in BP patients, but the trial was terminated in 2019 due to partial recruitment completion (146). DF2156A, an allosteric dual inhibitor of IL-8, was evaluated in a Phase 2 open-label trial (NCT01571895) for its effectiveness in improving BP-related bullous activity; however, the trial was prematurely discontinued, and no further studies have been conducted (147). The potential of targeting IL-17 and IL-23 in BP treatment is still under investigation, and these approaches are primarily used in BP patients with comorbid psoriasis (148). Ixekizumab and secukinumab, both IL-17 inhibitors, have shown efficacy in BP cases complicated by psoriasis (149–151), with secukinumab inducing long-term remission and reducing BP180-NC16A antibody levels (152). Ustekinumab and tildrakizumab, IL-23 inhibitors, have been reported to successfully treat a case of recurrent BP associated with psoriasis (153). However, there are also reports of new-onset BP in some psoriasis patients treated with ustekinumab (154). Kerkemeyer et al. successfully used tildrakizumab to treat refractory lichen planus pemphigoides, but clinical data supporting its use in BP remain insufficient (155). Moreover, Liu et al. (107)demonstrated that NE inhibitors prevent blister formation in NE-deficient and wild-type mice, as well as block DEJ separation in ex vivo human skin models. Williams et al. (156) reported that doxycycline (200 mg/day) effectively controls blisters by inhibiting MMPs and is safer than the standard treatment of oral prednisone (0.5 mg/kg/day). However, NE inhibitors (such as sivelestat) and MMP inhibitors (such as batimastat or andecaliximab) have not yet been used as treatments for BP (157). The exact effects of these treatment options in BP remain uncertain, and larger randomized controlled trials are needed to validate their efficacy and safety.

As previously discussed, some cytokines (IL-5, IL-4, IL-13, and IL-31) and complement factors (C5a and C3a) play crucial roles in the pathogenesis of BP by influencing eosinophils and basophils. Anti-IL-5 monoclonal antibodies, such as mepolizumab and reslizumab, target IL-5 to reduce eosinophil levels in the serum. While mepolizumab does not show an advantage over steroids in clinical trials (158), reslizumab has demonstrated significant effectiveness in treating BP (159).The anti-IL-4/-13 monoclonal antibody, dupilumab, which inhibits IL-4 and IL-13 signaling, has been successfully used in BP treatment, especially when combined with steroids or immunosuppressants, showing superior efficacy compared to steroids alone (160). Dupilumab also targets IL-31 and other pruritus-related signals, offering relief for BP patients with persistent itching (161). Studies have found that dupilumab can not only reduce the dosage of glucocorticoids in BP patients but, most importantly, it is highly safe for elderly patients with multiple comorbidities (162). Complement system inhibitors, such as avdoralimab and nomacopan, which inhibit C5aR1 and C5/LTB4 activity, have shown promising potential in BP treatment, with nomacopan demonstrating significant clinical efficacy (163, 164). Lastly, CD46 represents a novel therapeutic strategy aimed at providing protection against BP pathogenesis by inhibiting C3 deposition, with research supporting its potential clinical application (165). Overall, these immune-modulating antibody therapies offer promising new avenues for BP treatment, though further clinical trials are necessary to confirm their efficacy and safety.