Self-gelling nucleic acids have been developed, which gelate under physiological conditions without use of DNA ligase or any other crosslinking agents. A monomer unit consists of two or more oligonucleotides, each of which has two parts: one being complementary to other strands in the same unit and one (an adhesive end) complementary to other units. This unit, which can be called as a polypod-like structured nucleic acid, or polypodna, based on its structural characteristics, forms DNA hydrogel through hybridization of the adhesive ends. The conditions for gelation depend on the structure of polypodna, i.e., the pod number and the length of adhesive end, DNA concentration and the environmental factors, such as salt concentration. Any cargo, including small molecules, proteins, nucleic acids and cells, can be incorporated into and sustainedly released from DNA hydrogels. Immunostimulatory, immunosuppressive or immunomodulatory DNA hydrogels can be prepared by selecting the sequences of the components.