Antigens, a term for any foreign substance, like a virus or bacteria, are recognized by the body to induce an immune response to fight off infection. An antigen is a molecule capable of stimulating an immune response. They may be proteins, polysaccharides, lipids or nucleic acids. Each antigen has distinct surface features that are recognized by the immune system.

SARS-CoV-2, the virus that causes COVID-19, has several known antigens, including its nucleocapsid phosphoprotein and spike glycoprotein, which are the visible protrusions on its surface. An antibody is a Y-shaped protein produced by B cells of the immune system in response to exposure to antigens. The tip of each Y-shaped arm contains antigen binding sites (paratopes) that bind to a specific portion of the antigen’s surface (epitope). This binding helps to eliminate antigens from the body, either by direct neutralization or by “tagging” them for elimination by other arms of the immune system.