Building conventions for unconventional lymphocytes

Falling between the classical characteristics of innate immune cells and adaptive T and B cells are a group of lymphocytes termed "unconventional." These cells express antigen-specific T or B cell receptors, but behave with innate characteristics. Well-known members of this group include the gamma-delta T cell and the Natural Killer T cell. Recent literature has greatly expanded scientific knowledge of unconventional lymphocytes, but key questions remain unresolved in the field, including why these cells have been maintained concurrently with conventional innate and adaptive immune cells. Here, we summarize current literature that suggests what their unique purposes may be, including specialized functions with the microbiota and in early development. From the consensus literature, we discuss where we see unconventional lymphocytes fit into the logical organization of the complete immune system.