<b><i>Introduction:</i></b> Major depressive disorder (MDD) can impact the severity of allergic rhinitis (AR) and asthma (AA). Here, we evaluated the cytokine production by T-cells from AR and AA patients with or without MDD. The effect of serotonin on the in vitro T-cell response was also evaluated. <b><i>Methods:</i></b> The cytokines produced by activated T-cells were measured by Luminex and flow cytometry. In some cell cultures, serotonin was added. <b><i>Results:</i></b> MDD not only enhanced the production of Th2- and Th17-related cytokines, but also, the levels of interleukin (IL)-5 and IL-17 were directly correlated with the severity of depression and anxiety symptoms. As compared with AR, the levels of IL-17 were higher and the release of IL-10 was lower in activated T-cell cultures from AA patients, mainly those with MDD. In AA/MDD patients, the severity of anxiety symptoms and lung disease was directly correlated with Th17-like and hybrid Th2/Th17 cells, but inversely correlated with IL-10-secreting CD4⁺ T-cells. Finally, the addition of serotonin reduced the production of Th2- and Th17-related cytokines, but elevated IL-10 secretion in cell cultures from both AR and AA patients. <b><i>Conclusions:</i></b> Our findings suggest that not only the occurrence of MDD but also the severity of anxiety symptoms, may adversely affect the outcome of allergic reactions by favoring the production of cytokines implicated in the pathogenesis of AR and AA, a phenomenon that was attenuated by serotonin.

Background Obesity has often been associated with severe allergic asthma (AA). Here, we analyzed the frequency of different circulating CD4+T‐cell subsets from lean, overweight and obese AA patients. Methods Mononuclear cells from peripheral blood were obtained from 60 AA patients and the frequency of different CD4+T‐cell subsets and type 1 regulatory B cells (Br1) was determined by cytometry. The effect of obese‐related leptin dose on cytokine production and Treg cell function in AA‐derived CD4+ T cell cultures was evaluated by ELISA and 3H thymidine uptake, respectively. Leptin levels were quantified in the plasma by ELISA. According to the BMI, patients were stratified as lean, overweight and obese. Results AA severity, mainly among obese patients, was associated with an expansion of hybrid Th2/Th17 and Th17‐like cells rather than classic Th2‐like cells. On the other hand, the frequencies of Th1‐like, Br1 cells and regulatory CD4+ T‐cell subsets were lower in patients with severe AA. While percentages of the hybrid Th2/Th17 phenotype and Th17‐like cells positively correlated with leptin levels, the frequencies of regulatory CD4+ T‐cell subsets and Br1 cells negatively correlated with this adipokine. Interestingly, the obesity‐related leptin dose not only elevated Th2 and Th17 cytokine levels, but also directly reduced the Treg function in CD4+ T cell cultures from lean AA patients. Conclusion In summary, our results indicated that obesity might increase AA severity by favoring the expansion of Th17‐like and Th2/Th17 cells and decreasing regulatory CD4+T cell subsets, being adverse effects probably mediated by leptin overproduction.