A capsid is the protein shell that surrounds and protects the genetic material of a virus, assembled from structural units called capsomers. Capsids can adopt different geometries, most commonly icosahedral or helical conformations, and play a critical role in host cell recognition, receptor binding, and viral entry. In the context of gene therapy, capsid engineering is used to optimize the tissue tropism, immunogenicity, and transduction efficiency of viral vectors such as AAV and adenoviruses.
