We consider the new type of water-soluble stoichiometric interpolymer polyelectrolyte complexes (IPEC) consisting of macromolecules having different affinities to solvent and different densities of ionic groups along the chain. It was proposed that complexes of such macromolecules have a core-shell structure. The inner part of complex with a radius r contains monomer units of both macromolecules, while exterior part with a radius R consists exclusively of monomer units of hydrophilic macromolecules. With such complex organization, the external charged hydrophilic shell effectively protects the complex from precipitation and fusion with other complex assemblies. These theoretical conclusions allow to explain the experimental data on the stability of IPECs containing poly(L-lysine), poly(L-lysine citramide), poly(L-lysine citramide imide), and poly(acrylic acid), and can be of interest in the field of DNA-polycation complexes aimed at transfecting genes to cell nuclei.