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).