## 3 Phenotypic and Endotypic Differences among Children and Adults with AD  
The appearance of lesions and their localization in the course of atopic dermatitis varies depending on the age group. However, the basic features of AD--the presence of atopy and xerosis--are similar in both adults and children [46]. Most often, in infants and children, inflammation and lesions appear on the face, trunk and around the extensors of the lower and upper extremities, and in advanced cases even throughout the body. In adults, on the other hand, the lesions occur in the flexor region, neck, ears, around the eyes, and are more chronic in nature. A typical feature of the condition is dark red erythema on the face and neck [12; 32; 40]. From the data presented by the researchers in [47], it can be seen that 85% of AD cases manifest themselves before the age of five, thus indicating the greatest severity of symptoms among the younger population [47; 48]. The data presented are consistent with population-based studies conducted in the United States, where the prevalence of atopic dermatitis in children was 10.7% and in adults 7.2% [48]. atopic dermatitis can develop de novo in adulthood (about 25%), and in children AD can resolve before the age of five [11]. However, according to the authors [11; 49], one in five pediatric patients with AD will develop symptoms of atopic dermatitis in adulthood. The authors of [32; 40] report that a disorder of the immune system, which is not fully developed in the first weeks and months of life, is responsible for atopic lesions in younger patients. Initially, most T lymphocytes are immature and only gradually develop into memory cells [50]. Studies [50] show that immune changes, including T-cell activation and differentiation, occur more rapidly in children with AD. This indicates accelerated activation of these cells, faster differentiation into specific subsets (TH1, TH22, TH17) and the influence of environmental and genetic factors, as well as chronic skin inflammation [40; 51]. Studies conducted to evaluate lymphocyte levels in newborns with atopic dermatitis have shown low levels of TH1 lymphocytes and high levels of TH2 in cord blood [40; 51]. Epigenetic factors are responsible for the above condition, supporting the statement that atopic dermatitis is a typical multifactorial disease [40; 51] (Figure 1).  
Another study [40] on blood flow cytometry confirmed the delayed development of TH1 lymphocytes populating the skin of children with AD compared to healthy children. Increased blood concentrations of TH2 lymphocytes in both adults and children with AD are noted as a common feature [40].  
The authors of [40] indicate that the number of tight junctions between epithelial cells in the skin is reduced and the expression of lipid barrier genes is decreased in children with AD. According to the authors of the above study, this has the effect of exacerbating the epidermal barrier defect in patients with atopic dermatitis despite normal levels of filaggrin protein in the skin [40]. A comparative study was conducted for three age groups (18-40, 41-60 and over 61) of atopic dermatitis patients [40]. The study observed that with age  
Figure 2: “Itch-scratch-itch cycle” in the course of atopic dermatitis based on [1; 23; 43; 44; 45].  
there is a decrease in TH2/TH22 levels with a concomitant increase in TH1/TH17, with a normalized epidermal barrier, which is a differentiating feature for AD occurring in adults compared to that in children [32; 40]. Other differences in the course of AD in children and adults are presented in Table 3. The study [32; 40] and also the study conducted by the authors of [51] indicated that therapy that is tailored to the age and specific stage of the disease is of great importance.