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Follicular helper T cells (also known as T follicular helper cells and abbreviated as T FH), are antigen-experienced CD4 + T cells found in the periphery within B cell follicles of secondary lymphoid organs such as lymph nodes, spleen and Peyer's patches, and are identified by their constitutive expression of the B cell follicle homing receptor CXCR5. [1]
The T helper cells (T h cells), also known as CD4 + cells or CD4-positive cells, are a type of T cell that play an important role in the adaptive immune system. They aid the activity of other immune cells by releasing cytokines .
B cells can present antigens to a specialized group of helper T cells called T FH cells. If an activated T FH cell recognizes the peptide presented by the B cell, the CD40L on the T cell binds to the B cell's CD40, causing B cell activation. [16] The T cell also produces IL-4, which directly influences B cells.
The latter case induces recognition by antigen-specific Th2 cells or Tfh cells, leading to activation of the B cell through binding of TCR to the MHC-antigen complex. It is followed by synthesis and presentation of CD40L (CD154) on the Th2 cell, which binds to CD40 on the B cell, thus the Th2 cell can co-stimulate the B cell. [11]
Bcl-6 is a master transcription factor for the regulation of T follicular helper cells (T FH cells). Bcl-6 is expressed when the cytokines Il-6 and/or Il-21 are recognized; these cytokines can be produced by antigen presenting cells (APCs: B cells, dendritic cells, or macrophages) when activated.
In immunology, affinity maturation is the process by which T FH cell-activated B cells produce antibodies with increased affinity for antigen during the course of an immune response. With repeated exposures to the same antigen, a host will produce antibodies of successively greater affinities.
T cell exhaustion is a poorly defined or ambiguous term. [60] [61] There are three approaches to its definition. [60] "The first approach primarily defines as exhausted the cells that present the same cellular dysfunction (typically, the absence of an expected effector response).
T h 17 cells play an important role in maintaining mucosal barriers and contributing to pathogen clearance at mucosal surfaces; such protective and non-pathogenic T h 17 cells have been termed as T reg 17 cells. [2] They have also been implicated in autoimmune and inflammatory disorders.