**1.1 Definition, prevalence, and classification**

Celiac disease (CD) is a genetically determined immune-mediated disease, and individuals with CD have specific HLA haplotypes (DQ2 and/or DQ8) that trigger an immune response to gluten intake, leading to intestinal and clinical signs and symptoms [1, 2], besides other autoimmune-associated CD diseases, such as dermatitis herpetiformis [3], type 1 diabetes mellitus, Hashimoto's thyroiditis, and Sjögren syndrome [4]. Also, there are some genetic syndromes that may be CD associated such as Down syndrome [5, 6], Turner syndrome [7], and Williams syndrome [8].

As CD is one of the most well-elicited autoimmune diseases and one of the most common permanent food intolerances among humans [9], its prevalence in the general population from Europe, USA, and countries where the population is

predominantly of European origin is approximately 1% [2]. Prevalence is lower, ranging from 0.15 to 0.84%, in Latin American countries such as Brazil [10–14]. When Brasília city (Brazilian capital and population representation) is considered, since it is a city formed by people from all regions of the country, the prevalence found in the general population is 0.34%, considering 0.21% in adults and 0.54% in children [11].

The CD prevalence can still be related to the cereal consumption that contains gluten (mainly wheat) and to the distribution of predisposing HLA alleles in the population [15, 16]. Besides, the existence of genetic and environmental factors may influence the CD prevalence rate in a region [15]. Last but not least, we highlight the microbiota variability, the existence of intestinal infections, and socioeconomic conditions, which are also factors that may influence the CD development and prevalence [17, 63].

According to clinical signs and symptoms, laboratory and histopathological findings, which together have been called "clinical forms," CD can be classified into five distinct forms (**Table 1**).
