Innate and adaptive immunities are the two arms of the vertebrate defense system against pathogen infections. Toll-like receptors (TLRs) are one of the most important regulators of both innate and adaptive immune responses because they have the ability to detect pathogen-associated molecular patterns and danger-associated molecular patterns (DAMPs).

A detailed understanding of the inflammatory signaling pathways and molecular regulation of innate immunity can help in the design and development of new vaccines to treat and/or prevent a number of diseases. Identification of most human TLR native ligands has provided an excellent opportunity for the development of small molecule analogs of the natural ligands to be used as modulators with better pharmacodynamic and pharmacokinetic properties. Hence, small molecule TLR agonists are excellent vaccine adjuvants because they activate the innate immune response and lead to the induction of adaptive immune responses.