Introduction
The Coronavirus Disease 2019 (COVID-19) caused by severe acute respiratory syndrome corona virus 2 (SARS-CoV-2) has rapidly spread in China and around the world and is still on-going (Sohrabi, Alsafi et al. 2020). All people, including children are susceptible to SARS-CoV-2, and a rapid progression in morbidity and mortality rate due to human-to-human transmission of the virus are still reported (Lotfi and Rezaei 2020).
Although the pathogenesis of COVID-19 is not fully described, the interaction between the SARS-CoV-2 and the immune system is so complicated, and lymphopenia, hyper-inflammatory responses, and cytokines play an important role in the pathogenesis of COVID-19 (Saghazadeh and Rezaei 2020, Yazdanpanah, Hamblin et al. 2020).
Recent studies suggest that the SARS-CoV-2 infection may primarily affect T lymphocytes, particularly CD4+ T and CD8+ T cells, which might be highly involved in the pathological process of COVID-19 (Henry, de Oliveira et al. 2020, Wang, Hu et al. 2020).
CD4+ and CD8+ T cells play a fundamental role in controlling viral infections maintaining cellular and humoral immune responses. These cells are involved in humoral and cytotoxic immunity against viral infections. CD4+ T cells have multiple roles and are required to support CD8+ T cell responses. Moreover, these cells help to B cells to elicit antibody responses (Ng, Snell et al. 2013). CD8+ T cells have been shown to play a critical role in mediating viral clearance after acute viral respiratory infections, including respiratory syncytial virus, influenza A virus, and human metapneumovirus (Wang, Nie et al. 2020).
Among the laboratory features of COVID-19, a number of hematological abnormalities have been described (Henry, de Oliveira et al. 2020); however, no studies about lymphocyte subset analyses have been reported in children. Therefore, it is important to explain the characteristics of lymphocyte subsets in pediatrics with mild or severe COVID-19. This study aimed to evaluate the lymphocyte subsets in 55 laboratory-confirmed cases of SARS-CoV-2.