CD4 and COVID-19: In COVID-19 patients with severe respiratory failure (SRF), either macrophage-activation syndrome or a marked reduction in human leukocyte antigen-D related (HLA-DR) expression, along with a profound depletion of CD4 lymphocytes, CD19 lymphocytes, and natural killer (NK) cells, has been observed (Giamarellos-Bourboulis et al., 2020).