More specifically, influenza viruses express two surface antigens (foreign proteins)—hemagglutinin (H) and neuraminidase (N)—which trigger an immune response. The exact form of these antigens changes every now and then, resulting in the periodic emergence of new, more virulent influenza viruses with the potential to cause a pandemic. The surface of SARS-CoV-2 does not have these antigens. Rather, similar to other types of coronaviruses, its outer surface is studded with glycoprotein spikes, which give such viruses a crownlike, or coronal, appearance. Spike glycoproteins are responsible for triggering the immune response, and they carry out the critical function of enabling the coronavirus particle to enter cells, where it then replicates.