Abstract: A one-step method for assembly of delivery compositions for one or more antigens or therapeutic biologics is based on non-covalent affinity capture of molecules from solution using a biodegradable polymer having functional groups to which the affinity ligand binds. The polymer-bound affinity complex, which includes the molecule(s) of interest is then recovered from the reaction solution, for example, by size exclusion filtration, to yield the assembled delivery composition. The affinity ligand can be a monoclonal antibody or a metal affinity ligand with bound metal transition ion. The assembled delivery compositions can be formulated as polymer particles, which can then be lyophilized and reconstituted for in vivo delivery of the non-covalently complexed antigen(s) or therapeutic biologic(s) with substantial native activity.