Fig. 3

Early and late stage immune response to SARS-CoV-2 infection in young and elderly Individuals. During early stage of SARS-CoV-2 infection in the respiratory tract of young individuals the pathogen is easily recognized and different chemokines and cytokines are secreted which enables the recruitment of immune cells to the site of infection (Upper Panel I). These processes also increase hematopoesis as well as since young individuals have a large pool of naive T cells they can easily respond to new pathogens (Lower Panel I). In elderly individuals early stage of SARS-CoV-2 infection is associated with hypo-responsiveness, where the immune cells are not efficiently recruited to the site of infection (Upper Panel II), thymus involution in elderly leads to lower number of naïve T cells which minimize the control of novel pathogens (Lower Panel II). Late stage of SARS-CoV-2 infection in the young is characterized by controlling and eliminating the virus, tissue repair (Lower Panel III), blocking of cell recruitment to the site of infection, developing memory T cell, and developing anti- SARS-CoV-2 antibody (Lower Panel III). Whereas the late stage of SARS-CoV-2 infection in the elderly is characterized by hyper- responsiveness, where large number of immune cells are recruited to the site of infection (Lower Panel IV), the bone marrow turned to myelopoiesis (producing more myeloid cells) and excessive release of pro-inflammatory cytokines (especially IL-6) leads to cytokine storm which could lead to severe COVID-19 (which could include multi-organ failure) and death (Lower Panel IV)