The coat outside the nucleic acid of the virus is called capsid. Numerous protein molecules join together to form the capsid. Each protein molecule is called a capsomer. There are two types of capsomia. Pentamer and Hexamer. A pentamer is composed of a single cluster of 5 and a hexamer of 6 monomers. Protein molecules arrange to form helical helix and spherical polyhedron structures. ΦX174 virus is composed of 12, herpesvirus 162 and TMV 2130–2200 capsomeres. Apart from proteins, the capsid also contains lipids and proteins. The capsid may have thorns or spikes on the outside. The capsid acts as an antigen. It protects the nucleic acid and helps in attacking the host.