3.2.2. Antibody responses
During the 2 weeks after the onset of clinical symptoms, most of the
COVID-19 infected patients indicate high titers of IgM and IgG
antibodies [18]. Laboratory findings exhibited that the convulsant
plasma of the recovered individuals contain high volumes of neutralizing
antibodies [16], indicative of CD4+ T cell
response involvement [19], which has the potential to be appraised
as passive immunotherapy to improve the condition of critically ill
patients. It was also found that the extent of neutralizing antibodies
has direct relationship with the severity of the COVID-19 infection
[20]. More analysis revealed that the SARS-CoV2 S protein is the
most target of such neutralizing antibodies, which is contained the S1
and S2 subunits. The S2 is in the proximity of the viral membrane and
participates in cellular fusion while the S1 organizes farther away
containing the receptor binding domain (RBD) and attaches to the cognate
host ACE2 receptor [21]. Neutralizing antibodies in COVID-19
patients pursue two main goals: restraining the S protein-ACE2
interaction by targeting the RBD domain, and blocking membrane fusion by
binding to other regions of the S1 and S2 compartments [22, 23].
Also, the IgG2a antibodies against the N portion, as the most frequent
coronavirus protein, has been observed in the sera of COVID-19 patients
with potential Fc-mediated viral clearance instead of direct
neutralization [24]. Unbelievably, several studies discovered the
earlier peak of the anti-S protein IgA response before emerging the IgM,
although the underlying mechanisms in unknown [25]. Previous results
showed that more than 90% of healthy adults are seropositive for the
IgG against four common human coronaviruses (229E, NL63, OC43 and HKU1)
[26]. Such antibodies, like the antibody responses to SARS-CoV and
SARS-CoV2, largely disappear within a few months. Therefore, T cell
responses are likely to be more effective than antibody titers in
inhibiting coronaviruses re-infection [26].