Adaptive Immunity to SARS-CoV-2:
Adaptive immune response against viruses in general, is directed mainly by humoral immunity which secretes neutralizing antibodies to block the binding of the viruses to their receptors on the target host cells and hence prevent their internalization inside these cells. The other arm of the adaptive immunity is the cell mediated T cell response which is concerned with cytotoxic killing of the virus infected cells which leads to eradication of the infection (Fig 3). The antibodies produced by T-dependent germinal center interactions are high affinity antibodies making them more efficient in neutralizing the viruses in their extracellular stage; before their entry into the host cells or during virus budding to infect neighboring cells or after death of the infected cells. These antibodies neutralize the viral infection by attaching to the viral envelope or capsid antigens and then prevent the virus binding and internalization to the host cells, consequently, the cycle of viral infection and spread to the near cells is broken down. Among the neutralizing antibodies, the IgA immunoglobulins are the most effective for blocking the viral infection in respiratory and intestinal systems. Furthermore, these antibodies may also opsonize the viruses and initiates their phagocytosis. In addition, complement system may help the humoral immunity by opsonizing the viruses-antibodies complexes and promoting their phagocytosis as well as formation of membrane attack complexes (MAC) in the envelope of certain viruses leading to their lysis directly(25).
Once the virus succeeds in entering the cell, then, it is not accessible anymore to the neutralizing antibodies and they have no effect on the viral infection at this stage. Then it is the role of the cell mediated immunity to interfere, mainly the CD8 T cells which recognize the virus antigens expressed on the groove of MHC class I of the infected cells. This leads to the activation of CD 8 cytotoxic T lymphocytes (CTLs), usually with the help of T helper lymphocytes cytokines, and the costimulatory molecules expressed on the surface of infected cells in order to reach the full activation and clonal expansion process. The effector CTLs exert their cytotoxic actions against the viral infected cells leading to their killing and elimination of the infection. They may also eradicate the viral infecting cells by other ways such as activation of nucleases inside the target cells leading to degradation of the viral genome (25).