Prologue: The “Ready-to-Go” Lymphocytes of the Immune System

Abstract

In the Prologue of Part VII, the typical features of innate lymphoid cells and unconventional T cells with partial innate function are briefly introduced to the reader. So far, three groups of innate lymphoid cells have been classified and categorized based upon shared expression of surface markers, transcription factors, and effector cytokines. Innate lymphoid cells, compared to typical lymphoid cells, are characterized by three main features: (1) the absence of recombination-activating gene-dependent rearrangement of antigen receptors, (2) a lack of phenotypical markers of myeloid cells and dendritic cells, and (3) the particular lymphoid morphology. Unconventional T cells refer to lymphocytes with partial innate function and, thus, operate in the afferent phase of the immune response. These cells are characterized by semi-invariant, invariant, or even germline-encoded T cell receptors and include natural killer T cells, gammadelta T cells, and mucosal-associated invariant T cells. These cells have formerly been described as innate lymphocytes; sometimes they are also called “innate-like” T lymphocytes because they have several abilities typical of cells of the innate immune system. The characteristic feature of some of these T cell receptor-bearing T cell subsets assigning them to the family of innate immune cells is their equipment with certain natural killer cell markers. The reader is made aware of the fact that, in this part of the book, some of the effector responses of these two critical families of innate immune cells are depicted by highlighting mechanisms of their activation including the action of MAMPs and/or DAMPs.

As outlined in Part III, Sects. 8.4 and 8.5, the whole family of mammalian innate immune cells includes ILCs and unconventional T cells with partial innate function. So far, three groups of ILCs, that is, ILC1, ILC2, and ILC3, have been classified and categorized based upon shared expression of surface markers, transcription factors, and effector cytokines. Innate lymphoid cells, compared to typical lymphoid cells, are characterized by three main features: (1) the absence of RAG-dependent rearrangement of antigen receptors, (2) a lack of phenotypical markers of myeloid cells and DCs, and (3) the particular lymphoid morphology. Unconventional T cells refer to lymphocytes with partial innate function and, thus, operate in the afferent phase of the immune response. These cells are characterized by semi-invariant, invariant, or even germline-encoded TCRs and include NKT cells, γδ T cells, and MAIT cells. These cells have formerly been described as innate lymphocytes; sometimes they are also called “innate-like” T lymphocytes because they have several abilities typical of cells of the innate immune system. The characteristic feature of some of these TCR-bearing T cell subsets assigning them to the family of innate immune cells is their equipment with certain NK cell markers. In Part VII of this book, some of the effector responses of these two critical families of innate immune cells are depicted by highlighting mechanisms of their activation including the action of MAMPs and/or DAMPs.