Follicular B helper T cells (also known as just Follicular helper T cells or T_FH), are antigen-experienced CD4⁺ T cells found in the periphery within B cell follicles of secondary lymphoid organs such as lymph nodes, spleens and Peyer's patches, and are identified by their constitutive expression of the B cell follicle homing receptor CXCR5. Upon cellular interaction and cross-signaling with their cognate follicular (Fo B) B cells, T_FH cells trigger the formation and maintenance of germinal centers through the expression of CD40 ligand (CD40L) and the secretion of IL-21 and IL-4. T_FH cells also migrate into these seeded germinal centers, predominantly composed of rapidly dividing and mutating B cells. Within germinal centers, T_FH cells play a critical role in mediating the selection and survival of B cells that go on to differentiate either into special plasma cells capable of producing high affinity antibodies against foreign antigen, and memory B cells capable of quick immune re-activation in the future if ever the same antigen is re-encountered. T_FH cells are also thought to facilitate negative selection of potentially autoimmune-causing mutated B cells in the germinal center. However, the biomechanisms by which T_FH cells mediate germinal center tolerance are yet to be fully understood.

Tetrahydrofuran (THF) is an organic compound with the formula (CH₂)₄O. The compound is classified as heterocyclic compound, specifically a cyclic ether. It is a colorless, water-miscible organic liquid with low viscosity. It is mainly used as a precursor to polymers. Being polar and having a wide liquid range, THF is a versatile solvent.