The cephalochordate amphioxus is the closest living invertebrate relative of the vertebrates and therefore it can provide useful insights on the evolution of the adaptive immune system. In fact some components of the immune system are present in amphioxus but many other features are lacking. A proto-MHC region has been identified by chromosome walking and the presence of T cell receptor has been demonstrated despite the absence of true lymphocytes. Moreover a further important step on the study of the amphioxus immune system is represented by the recent availability of the genome of Branchiostoma floridae that will allow to better understand how many genes code for immunological molecules involved in adaptive immune system pathways of amphioxus.
