Toll-Like Receptor Family Members and Their Ligands

On occasion, the innate immune system is referred to as the "primitive" immune system. Perhaps this has dissuaded immu­ nologists from analyzing it as energetically as they have analyzed the adaptive immune system during the past two decades. But while its phylogenetic origins are indeed ancient, and though it is "of the first type", there is nothing crude, nothing unsophisti­ cated, and nothing "inferior" about innate immunity. On the contrary, the innate immune system has had time to achieve a level of refinement that is nothing short of dazzling, and a modicum of respect is at long last due. Any immune system has two cardinal functions. It must destroy a broad range of pathogens, and it must spare the host. The adaptive immune system has applied a modular solution to these problems. Each cell of the adaptive immune system is prescreened to eliminate those that would produce untoward interactions with self; each cell is pre-programmed to recognize a foreign epitope that the host might one day encounter. Hence, the duties of each individual lymphocyte are quite circumscribed.