The vertebrate immune system reacts to certain sequences of DNA with a strong T(H)1-inducing innate response. These sequences, termed immunostimulatory DNA sequences, are not fully defined but generally consist of a central nonmethylated CG dinucleotide, flanked by less highly conserved sequences (hence the alternate name CpG motifs). These sequences seem to be rare in vertebrates but relatively common in many lower organisms, including bacteria and viruses. It is likely that these sequences represent a danger signal to the immune system; a powerful T(H)1 response is induced against colocalized foreign antigen. This can be used to modify an allergic response away from a pathogenic T(H)2-dominated immune profile toward a nonpathogenic and even protective T(H)1 profile.