Invertebrate immune recognition, natural immunity and the evolution of positive selection.

Invertebrate immunity exhibits immediate recognition without prior contact. Here, Tom Humphreys and Ellis Reinherz propose that receptors capable of actively recognizing self histocompatibility antigens could be generated by a process similar to positive selection, and that these could guide 'natural immunity' to foreign cells. These ideas provide a conceptual basis for invertebrate immunity and for understanding the evolutionary precedents for T-cell-mediated antigen recognition.