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].