The immune system appears to have evolved a mechanism to detect prokaryotic nucleic acids based on their content of unmethylated CpG dinucleotides in particular base contexts. This response promotes the activation of antigen-specific B cells and the secretion of T helper-1-like cytokines. Preliminary animal studies indicate the potential therapeutic utility of CpG DNA as a vaccine adjuvant, and for the immunotherapy of cancer and allergic diseases. On the other hand, CpG DNA can also serve as a trigger for the sepsis syndrome and may have a pathogenic role in certain inflammatory immune-mediated diseases.