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A mature virus particle, called a virion, contains a genome core with DNA or RNA surrounded by a protective protein coat called the capsid. The capsid is composed of protein subunits called capsomeres. Together, the capsid and genome form the nucleocapsid, which delivers genetic material into host cells for viral replication.

Icosahedral capsids are 20-sided, quasi-spherical structures found in viruses like rhinovirus. Helical capsids are thin, linear, rod-shaped structures where the nucleic acid genome fits inside grooves, as seen in tobacco mosaic virus. Both designs protect the genome but differ fundamentally in geometry and how genetic material is packaged.

Capsid proteins recognize and bind to specific cell membrane proteins on host cells, enabling infection. They also facilitate uncoating of the viral genome to allow replication inside the host. Additionally, capsid proteins are unique to viruses and serve as microbe-associated molecular patterns that trigger immune responses.

Enveloped viruses have a lipid and protein envelope surrounding the capsid, acquired from the host cell membrane. Non-enveloped viruses lack this outer layer, so the nucleocapsid is the complete virion. The viral envelope protects the virus and mediates interactions with host cells, while non-enveloped viruses rely solely on capsid proteins.

Capsid and envelope proteins are unique to viruses and function as microbe-associated molecular patterns that the immune system recognizes. In plants, these patterns trigger cascades of immune responses that mitigate current infections and prepare for future pathogen encounters. In humans, they induce innate immune responses including inflammation and antimicrobial protein production.

The protein subunits called capsomeres that compose the capsid dictate the overall size of a virus particle. These capsomeres assemble into specific geometric structures, such as icosahedral or helical shapes, which determine how much genetic material can be packaged and the final dimensions of the virion.

Some vaccines take advantage of the body's ability to recognize microbe-associated molecular patterns present on viral capsids and envelopes. By exposing the immune system to these unique viral proteins, vaccines enable recognition and response to specific viral pathogens without causing infection, providing protective immunity.