## 1 Introduction  
Pustular psoriasis (PP) and related pustular skin disorders are immune-mediated diseases consisting of several clinical entities including localized pustular psoriasis, mainly palmoplantar pustular psoriasis (PPP) and acro dermatitis continua of Hallopeau (ACH), generalized pustular psoriasis (GPP), a life-threatening form of psoriasis, and acute exanthematous generalized pustular eruption (AGEP), a severe subtype of drug hypersensitivity [1]. These entities are recognized by the presence of multiple pustules as a result of extensive neutrophilic accumulation [1; 2; 3]. The sterile pustular eruptions noted in 20-30% of patients with adult-onset immunodeficiency (AOID) are particularly noteworthy. AOID is an acquired form of immunodeficiency, occurring in adults who previously had normal immune function. This condition arises due to the development of autoantibodies against interferon-\(\gamma\) (IFN-\(\gamma\)), a crucial cytokine in immune response. The resultant immunodeficiency in AOID patients manifests similarly to AIDS, with patients being susceptible to a range of opportunistic infections (OIs) typically seen in those with severely compromised immune systems [4]. This phenomenon has been extensively studied, with reactive neutrophilic dermatoses being identified as a common feature in AOID due to the anti-IFN-\(\gamma\) autoantibody [2]. Furthermore, a variety of reactive and infective dermatoses, including Sweet's syndrome, have been associated with AOID [5]. Since this pustular reaction in AOID is phenotypically similar to other pustular skin disorders (Figure 1), the terms \"PP\", \"AGEP\", \"exanthematous pustulosis\", \"generalized pustular eruption\", and \"subcorneal pustulosis\" have been used variably and interchangeably in this condition [3; 4; 6; 7; 8; 9].  
While the biological mechanism of this similar phenotype remains unknown, both AOID and PP are thought to be multifactorial diseases involving pathogenic mutations in multiple genes in combination with environmental factors that could constitute the initial trigger of inappropriate immunity activation. To the best of our knowledge, genetic abnormalities in _IL36 RN_, _CARD14_, and _AP153_ genes have been proposed to play an important role in pathophysiological features in PP, but the high genetic heterogeneity of the diseases leads to ambiguous conclusions [1]. It has been estimated that the abnormalities in _IL36 RN_, _CARD14_, and _AP153_ account for fewer than 30% of the PP cases; therefore, additional genetic factors of the diseases remain to be elucidated [10]. Recently, a missense mutation in the predisposing gene, _SERPINA1_, has been identified in two unrelated patients, one with AOID who has pustular skin reaction and the other with PP, providing the first evidence of the crucial link between these two diseases [11].  
A few genetic variations of the human leukocyte antigen (HLA), a set of genes encoding for cell-surface molecules responsible for the regulation of the immune system, either class I or II, have been identified in association with PP in the Japanese population [12; 13], while _HLA_ allele variants, mainly of class II, such as _DRB1*16:02_ and _DQBI*05:02_, have been associated with AOID in Taiwanese [14], Chinese [15], and more recently, Thai [16] populations. Despite these observations suggesting that HLA genes could play a critical role in the pathogenic mechanism of both AOID and PP, the role of HLA in the pustule formation remains unknown, so far. In this study, we performed the HLA genotyping of a Thai nationality cohort presenting multiple sterile skin pustules and compared it with the available dataset of a healthy unrelated Thai population to assess the genetic burden of HLA variants in the manifestation of these pustular skin diseases.