CD4 and COVID-19: COVID-19 has been called a serial lymphocyte killer because profound and protracted lymphopenia is a near uniform finding among patients with severe COVID-19 and correlates with morbidity and mortality.1,3 Autopsies demonstrate a devastating depletion of lymphocytes in the spleen and other organs.2 CD4, CD8, and natural killer cells, which play important antiviral roles, are depleted and have reduced function, leading to immune collapse.1