This association may be explained by the findings that males tend to have lower serum IgG antibody generation, decreased CD4+ T cell reserves, and lower circulating ACE2 expression when compared to females.  Male sex is also found to be associated with increased disease severity upon hospital admission, higher rates of ICU admission, and increased clinical markers such as lymphopenia and inflammatory indexes. Conversely, female sex is found to play a significant role in lowering risk of mortality from COVID-19. Here, CD4 is linked to COVID-19.