When foreign pathogens invade the human body, the immune system initiates and induces a cascade of immune responses to clear the pathogens. Adaptive immunity, including humoral and T cell-mediated immunity, plays a critical role in the elimination of pathogens, including SARS-CoV-2. Cytotoxic lymphocytes (primarily cytotoxic CD8+ T cells) can eliminate infected cells, and specific antibodies against SARS-CoV-2 in the humoral immune response have the potential to neutralize this virus or even help cytotoxic T cells eliminate virus-infected cells to control disease progression (Dörner and Radbruch,2007; Liu et al., 2017).

Under stimulation by SARS-CoV-2 antigens, B cells from germinal centers can proliferate and differentiate into plasma cells, producing and secreting specific antibodies to control viral replication, Virions may also directly modulate host specific immunity by infecting immune cells expressing the viral receptor ACE2, such as pulmonary monocytes and macrophages. These antibodies can be present in the blood or produced de novo by memory B cells and plasma cells upon re-exposure to viral antigens. Thus, SARS-CoV-2-specific humoral immunity plays a critical role in antiviral defense by providing newly produced antibodies from activated plasma cells (Palm and Henry,2019).

Similar to other respiratory infections, SARS-CoV-2 infection stimulates rapid production of IgM, IgG and IgA antibodies, which are measurable in the sera as early as a week post-symptom onset, which are especially directed against nucleocapsid (N) and spike (S) proteins (Tan et al., 2004). N is the virus nucleocapsid protein, is highly immunogenic and abundantly expressed in vivo after the virus infects human being. S is responsible for binding the virus to the human angiotensin converting enzyme 2 (ACE2) and its subsequent cellular uptake (Meyer et al., 2014). The stability of IgG levels persisted over an extended duration, whereas a decline in IgM levels commenced at an earlier stage (Li et al., 2008).

SARS-CoV infection induces seroconversion as early as day4 after onset of disease and was found in most patients by 14 days. Long lasting specific IgG and neutralizing Antibody are reported as long as 2 years after infection (Zhou et al., 2020a). For MERS-CoV infection, seroconversion is seen at the second or third week of disease onset. For both types of coronavirus infections, delayed and weak Antibody response are associated with severe outcome (Liu et al., 2017). In a preliminary study, one patient showed peak specific IgM at day after disease onset and the switching to IgG by week 2.25. Furthermore, all sera from patients were able to neutralize SARS-CoV-2 in an in vitro plaque assay, suggesting a possible successful mounting of the humoral responses (Liu et al., 2006).

Increased both IgM and IgG titers of antibodies which found in serum patients infected with severe illness, the available data suggests that an effective Antibody reaction is associated with increased severity of sickness (Tan et al., 2020). Apart from antibodies that neutralize, that has protective and advantageous properties, the system has a large number of non-neutralizing antibodies, which aid in immune system cells and APCs that are becoming infected (Iwasaki and Yang ,2020). Additionally, a number of sources of evidence demonstrate the impact of severe respiratory disease in COVID-19 patients coincident with antiviral IgGs (Zhou et al., 2020a).

References
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