Several mechanisms likely contribute to the reduced number of T cells in the blood, including effects from the inflammatory cytokine milieu. Indeed, lymphopenia seems to correlate with serum IL-6, IL-10, and TNF-α (Diao et al., 2020, Wan et al., 2020a), while convalescent patients were found to have restored bulk T cell frequencies paired with overall lower proinflammatory cytokine levels (Chen et al., 2020f, Diao et al., 2020, Liu et al., 2020a, Liu et al., 2020b, Zheng et al., 2020b). Cytokines such as IFN-I and TNF-α may inhibit T cell recirculation in blood by promoting retention in lymphoid organs and attachment to endothelium (Kamphuis et al., 2006, Shiow et al., 2006). However, in an autopsy study examining the spleens and hilar lymph nodes of six patients who succumbed to COVID-19, Chen et al. observed extensive cell death of lymphocytes and suggested potential roles for IL-6 as well as Fas-FasL interactions (Chen et al., 2020h). In support of this hypothesis, the IL-6 receptor antagonist tocilizumab was found to increase the number of circulating lymphocytes (Giamarellos-Bourboulis et al., 2020). T cell recruitment to sites of infection may also reduce their presence in the peripheral blood compartment. scRNA-seq analysis of bronchoalveolar lavage (BAL) fluid of COVID-19 patients revealed an increase in CD8 T cell infiltrate with clonal expansion (Liao et al., 2020). Likewise, post-mortem examination of a patient who succumbed to ARDS following SARS-CoV-2 infection showed extensive lymphocyte infiltration in the lungs (Xu et al., 2020c). However, another study that examined post-mortem biopsies from four COVID-19 patients only found neutrophilic infiltration (Tian et al., 2020a). Further studies are therefore needed to better determine the cause and impact of the commonly observed lymphopenia in COVID-19 patients.