CD4 and macrophage activation syndrome: ARDS/severe COVID-19 pneumonia is often accompanied by the macrophage activation syndrome (MAS) or an IL-6-mediated very low human leukocyte antigen D related (HLA-DR) expression; a profound depletion (lymphopenia) of CD4 lymphocytes, CD19 lymphocytes, and natural killer (NK) cells; and hyper-inflammation [19].