[New aspect of immune system: innate immunity and acquired immunity]

Recently, it has been turned out that our internal defense system is composed of two distinct components; innate/natural immune system and acquired/adaptive immune system. The former innate immunity is principally located at the surface area such as skin and mucosal compartment, while the latter acquired immunity is observed mainly in the circulating blood and lymphoid organs. The critical difference between those two systems exists in the receptors as well as their ligands. Rearranged gene-derived receptors like immunoglobulin (Ig) and MHC molecule-restricted alphabeta-type of T-cell receptors (TCR) with high specificities and memories are used to recognize peptide antigens in the acquired immunity, whereas non-rearranged invaliant receptors such as toll-like receptors (TLR), gammasigmaTCR and CD1 molecule-restricted alphabeta TCR are employed to detect lipid/glycolipid or nucleic acid-related antigens in the innate immunity. Based on such new findings, the actual roles of immunity are discussed.