CD8A and COVID-19: Another group used single-cell sequencing technology to analyze the immune cell subtypes in the bronchoalveolar lavage (BAL) fluid of COVID-19 patients with different disease severities and found clonal CD8+ T-cell proliferation in the alveoli of patients with mild disease and a disruption in T-cell subset distribution in severe patients, suggesting that after SARS-CoV-2 infection, the human body produces a memory immune cell response against SARS-CoV-2 and that a virus-specific CD8+ T-cell immune response alleviates symptoms rather than aggravating a patient’s condition [57].