Complement components are present in invertebrates as elements of an innate immune system.An alternative adaptive immune system has been described recently in the most basal vertebrates, lampreys and hagfish, which are the only surviving jawless vertebrates.These eel-like fish use leucine-rich-repeat (LRR)-based receptors, called variable lymphocyte receptors (VLRs), for antigen recognition instead of the immunoglobulin (Ig)-based receptors used in vertebrates.