Primordial Cells Bridging the Gap Between Innate and Adaptive Immunity.

Rapid reactions to the presence of non-self through the activation of germline-encoded receptors are a hallmark of the innate immunity. Lymphocytes are an integral component of the immune system and responsible for the specificity of adaptive immunity. These cells are the front-line defense of the immune system and have a wide array of functions that range from producing antibodies, having memory regulating inflammation, and elimination of transformed/malignant cells. Falling between the classical characteristics of innate immune cells and adaptive T and B cells is a group of lymphocytes termed unconventional lymphocytes. They are highly diverse group of cells and overlap between the typical features of innate and adaptive immune cells. They exhibit innate immune characteristics, but they also express T or B cell receptors specific to antigens. It includes natural killer (NK) cells lacking variable antigen receptors of T and B cells, γδT cells, natural killer T (NKT) cells, mucosal-associated invariant T cells (MAIT) cells, and intraepithelial lymphocytes (IELs) belonging to T cells expressing αβ or γδ TCRs. The list also includes B cell receptor expressing B 1 cells and marginal zone B cells. This chapter summarizes the role of specialized lymphocytes that act as key sensors and their immune response under various conditions of stress.