Immunology Terms Starting With F
Immunology Glossary: F
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Fas Ligand
/ FAS LY-gand / · Fas is an acronym from Fibroblast-associated surface antigen; ligand from Latin ligare, meaning to bind
Fas Ligand is a transmembrane protein of the tumor necrosis factor family that induces apoptosis by binding to the Fas death receptor on target cells.
Fas ligand is expressed primarily on activated cytotoxic T cells, natural killer cells, and cells in immune-privileged sites such as the eye and testis, where it eliminates autoreactive lymphocytes and maintains tissue integrity. When FasL binds its receptor Fas, also known as CD95, it triggers formation of the death-inducing signaling complex, which activates caspase-8 and initiates the apoptotic cascade within minutes. Humans lacking functional FasL develop autoimmune lymphoproliferative syndrome, characterized by massive lymphocyte accumulation and systemic autoimmunity.
Cytotoxic T cells use Fas-FasL interactions to kill virally infected or cancerous cells, complementing the perforin-granzyme killing mechanism. Beyond cell death, FasL can also deliver non-apoptotic signals that promote inflammation and tissue remodeling depending on cellular context and receptor density.
Some cancer cells exploit Fas ligand by expressing it on their surface, killing infiltrating immune cells in a process called tumor counterattack. This immune evasion strategy was first described in human colon carcinoma tissue by Hahne and colleagues in 1996.
Fas ligand does not only cause cell death. At low concentrations or in specific cell types, FasL can promote cell survival, proliferation, or differentiation depending on which receptor complexes form.
In acute hepatitis caused by hepatitis B virus, excessive Fas-FasL interactions between infiltrating immune cells and hepatocytes drive widespread liver cell death. Some patients lose more than 50 percent of functional liver tissue within days through this apoptotic pathway, a scale of destruction that perforin-granzyme killing alone cannot account for.
Fc Receptor
/ ef-see reh-SEP-tor / · Fc: Fragment crystallizable region + Latin receptare
Fc Receptor is a cell-surface protein on immune cells that binds to the constant region of an antibody, linking antibody recognition of a target to direct cellular responses such as phagocytosis, degranulation, or cytotoxicity.
Five major classes of Fc receptors exist, each with distinct cellular distribution and functional outcomes. FcgammaRI on macrophages and dendritic cells promotes phagocytosis of IgG-coated bacteria, while FcepsilonRI on mast cells and basophils triggers degranulation when IgE-bound allergens cross-link the receptor. FcRn, the neonatal Fc receptor, extends IgG half-life by rescuing antibody from lysosomal degradation and recycling it to the cell surface, giving IgG a serum half-life of roughly 21 days in humans.
Natural killer cells express FcgammaRIII, which mediates antibody-dependent cellular cytotoxicity against IgG-coated tumor cells and virus-infected cells. Mutations or polymorphisms in Fc receptor genes influence susceptibility to infections, autoimmune diseases, and responses to therapeutic monoclonal antibodies.
Therapeutic antibodies such as rituximab and trastuzumab are deliberately engineered to optimize Fc receptor binding on natural killer cells and macrophages, improving their ability to destroy cancer cells through antibody-dependent cellular cytotoxicity. Some next-generation antibody drugs carry Fc mutations that increase FcgammaRIII affinity by more than tenfold compared to unmodified IgG.
Immune System Fun Facts →Antibodies work alone after binding a target. Fc receptors connect antibody-coated targets to cellular effector functions, meaning macrophages, neutrophils, and natural killer cells must engage the antibody's constant region to execute phagocytosis or killing.
In the house mouse (Mus musculus), FcgammaRIII on splenic natural killer cells binds IgG-coated tumor cells and triggers antibody-dependent cellular cytotoxicity. Experiments in FcgammaRIII-knockout mice show tumor clearance drops by more than 80 percent compared to wild-type animals, demonstrating how much this single receptor contributes to antibody-mediated tumor surveillance.
FGL1
/ ef-jee-el-WUN / · FGL1: Fibrinogen-Like Protein 1
FGL1 is a liver-secreted fibrinogen-like protein that binds the LAG-3 immune checkpoint receptor on T cells, suppressing T cell activation and providing tumors with a mechanism of immune evasion independent of the PD-1 pathway.
FGL1 was identified in 2019 by Weiping Zou’s group as a principal LAG-3 ligand distinct from MHC class II, establishing a non-MHC pathway through which LAG-3 suppresses T cell responses. Hepatocytes produce FGL1 under normal conditions, but many tumor types upregulate its expression, allowing cancer cells to suppress tumor-infiltrating T cells by engaging LAG-3 in the tumor microenvironment. High plasma FGL1 concentrations in cancer patients correlate with poor response to anti-PD-1 checkpoint therapy and reduced survival, positioning FGL1 as both a prognostic biomarker and a therapeutic target.
Blocking the FGL1-LAG-3 interaction with antibodies restores T cell proliferation and cytokine production in preclinical models, supporting the development of combination checkpoint blockade strategies.
Because FGL1 is produced by the liver under normal physiological conditions, healthy individuals carry measurable baseline plasma FGL1 levels. Patients with hepatocellular carcinoma show FGL1 plasma concentrations several times higher than healthy controls, suggesting the liver tumor itself amplifies the very suppressive signal that shields it from T cell attack.
Every checkpoint ligand is found only on tumor cells. FGL1 is secreted by normal hepatocytes and circulates in the blood, meaning T cells can encounter FGL1-mediated suppression both within tumors and in the systemic circulation.
In mouse models of colorectal cancer, tumors engineered to overexpress FGL1 grow significantly faster than control tumors and show reduced infiltration by CD8 T cells. When researchers treated these mice with an anti-LAG-3 antibody, intratumoral CD8 T cell numbers recovered and tumor growth slowed by approximately 50 percent compared with untreated controls.
Follicular Helper T Cell
/ foh-LIK-yoo-lar HEL-per TEE sel / · From Latin folliculus, meaning small bag, referring to lymphoid follicles where these cells reside
Follicular Helper T Cell is a specialized CD4-positive T lymphocyte that migrates into B cell follicles of secondary lymphoid organs and drives germinal center reactions that produce high-affinity, class-switched antibodies.
Follicular helper T cells express the transcription factor Bcl-6 and the chemokine receptor CXCR5, which directs their migration into B cell follicles within secondary lymphoid organs. These cells interact with B cells in germinal centers, delivering signals through CD40 ligand, IL-21, and IL-4 that promote B cell survival, class switching, and somatic hypermutation. A single germinal center typically contains 100 to 300 follicular helper T cells that compete to provide help to antigen-specific B cells.
Dysregulation of these cells contributes to autoimmune diseases such as systemic lupus erythematosus, where excessive follicular helper T cell activity drives pathogenic autoantibody production, and to immunodeficiency when their function is impaired. Their formal identification as a distinct lineage around 2000 resolved the longstanding question of how T cells assist B cells despite being largely excluded from follicular regions.
Follicular helper T cells are a preferred hiding place for HIV because germinal centers have lower concentrations of cytotoxic T cells than surrounding tissue, allowing viral replication to continue even when antiretroviral therapy suppresses virus elsewhere in the body. This sanctuary effect makes germinal center reservoirs one of the main obstacles to a functional HIV cure.
Follicular helper T cells are the same as conventional helper T cells, differing only in location. Follicular helper T cells are a distinct lineage defined by Bcl-6 expression, CXCR5-driven follicular homing, and the specialized capacity to sustain germinal center reactions that generate high-affinity antibodies.
In immunized sheep (Ovis aries), follicular helper T cells accumulate in the large popliteal lymph nodes within 7 to 10 days after antigen injection into the hind limb. Germinal centers in these nodes can expand to occupy more than 30 percent of the follicular area at peak response, a scale that makes sheep lymph nodes a practical model for studying follicular helper T cell dynamics.