CD4 and COVID-19: Except for macrophages and type I and type II pneumocytes, all other immune cells (B cell, CD4 + T cell, CD8 + T cell, dendritic cell, monocyte, natural killer cell, neutrophil and T cell: gamma–delta) were dominantly differentiated and chemotactic in the BALF of COVID-19 patients compared to healthy volunteer (Fig. 4C).