Abstract: The present invention provides for a protein cage polypeptide (or scaffolding protein) useful or capable of forming a hollow tetrahedral pyramid structure, and a “self-assembling protein nanoparticle decorated with antibodies” (SAPNA) which is a chimeric protein assembly comprising: (a) one or more antibodies and (b) the protein cage polypeptide that provides a scaffold upon which to array the antibodies. In some embodiments, the antibody is capable of binding specifically to a pathogenic biological agent, or part thereof.