Antibodies, or immunoglobulins, are protective proteins produced by the immune system in response to foreign substances. They recognize and bind to antigens, facilitating their removal from the body. There are five classes of antibodies (IgM, IgD, IgG, IgA, and IgE) distinguished by their constant regions. IgG antibodies against SARS-CoV-2 can be detected after 8 days of symptom onset, with a high seropositivity rate after 14 days. In a small study, some COVID-19 patients tested negative for IgG antibodies against one antigen but positive against another.