Bacterial DNA and oligonucleotides containing unmethylated CpG dinucleotides (CpG DNA) can stimulate immune responses and have potential for use as novel agents to enhance immunogenicity. CpG DNA can interact with toll-like receptor 9 and cause activation through a myeloid differentiation primary response gene (MyD88)-dependent signaling pathway. Due to its pattern of immune cell activation, CpG DNA can induce a cytokine milieu to promote T-helper cell responses and serve as an adjuvant. Furthermore, CpG DNA can provide protection against pathogens in animal models and has therapeutic applications in clinical settings such as in cancer and allergy.