The immune system is broadly divided into the innate and adaptive immune systems.
Innate immune responses are the first line of defense against viruses and are triggered when cellular pattern recognition receptors, such as toll-like receptors, recognize pathogens associated molecular patterns.
Innate antiviral immunity includes: type 1 interferons, antiviral cytokines, and cellular responses of neutrophils, monocytes, macrophages, dendritic cells, and natural killer cells.
The second line of defense against viruses, the adaptive immune response, involves antigen specific recognition of viral epitopes.
The adaptive immunity immune system includes: humoral immunity and cellular immunity.
Humoral immunity to viruses includes antibodies that bind the spike proteins and either neutralize the virus or eliminate it through other effective mechanisms.
Cellular immunity to viruses includes virus specific B cells and T cells, which provide long-term immunologic memory and rapidly expand on the exposure to antigen.
B cells produce antibodies, CD8 positive T cells directly eliminate virally infected cells, and CD4 positive T cells provide help to support the immune system.
For acute viral infections it is likely that neutralizing antibodies are critical for blocking acquisition of infection, whereas a combination of humoral and cellular immune response is most likely controls viral replication after infection and prevents progression to severe disease, hospitalization, and death.
For highly transmissible viral infections that largely escape neutralizing antibodies, cellular immunity may be important for a long time protection against severe disease.