Abstract: Polyelectrolyte nanoparticle compositions for biomedical applications are provided comprising at least two carrier domains comprising multivalent ionic domains and an agent exhibiting biological activity when contained within the nanoparticle or on the nanoparticle surface. The multivalent ionic domains may be contained in two separate molecules or in separate but linked domains of a single molecule. The nanoparticle optionally can further comprise an exposed targeting ligand and/or protective surface. The nanoparticle can be contacted to cells or administered directly to an animal for biomedical applications including therapeutics and immune response. The nanoparticle may alternatively be comprised of a carrier material capable of delivering various medically important antigens as vaccine.