Adaptive immunity: Based on the dual recognition responses of αβT cells.

It is proposed that the recently reported second recognition mode of αβT cells allows an explanation of the evolutionary origin of adaptive immunity. The dual modes provide the mechanism of development/differentiation of αβT cells under the control of immune response (Ir) genes by which a given T-cell can distinguish one kind of MHC molecule (as 'self' phase, learned from its positive selection) from others (as 'nonself' phase) involved in antigen recognition. It is thus possible to re-explain the 'self'/'nonself' concept based on 'homotype (phase)'/'heterotype (phase)' recognition at the level of a single clone of T cells. Hence adaptive immunity is explained here as being derived from functions that ensure synchronous ontogeny and prevent the paradoxical (or retrograde) development of vertebrates.