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Fab  The part of an antibody molecule which contains the antigen-combining site, consisting of a light chain and part of the heavy chain; it is produced by enzymatic digestion.

Facilitated diffusion  Protein-aided transport of an ion or molecule across a membrane down its concentration gradient. This type of transport is characterized by three properties: (1) a rate faster than that predicted by passive diffusion through a phospholipid bilayer according to Fick’s law; (2) ligand specificity (expressed by Km); and (3) saturation (Vmax). The glucose transporter is a well studied example of a protein that mediates facilitated diffusion.

Factors B, P, D, H, and I  Components of the alternative complement pathway

Factor XII  Initial factor in complete intrinsic pathway sequence of reactions that results in blood clotting.

Factor XIIIa  Activated plasma protein that catalyzes cross-link formation between fibrin molecules to strengthen blood clot.

Fc  The portion of an antibody that is responsible for binding to antibody receptors on cells and the C1q component of complement.

Fc receptors  Surface molecules on a variety of cells that bind to the Fc regions of immunoglobulins. They are antibody class specific and isotype selective

Feedback inhibition  Decrease in the catalytic activity of one of the enzymes in a metabolic pathway caused by binding of the ultimate product of the pathway. Usually, the inhibited step is the first step in the pathway that does not lead to other products.

Fermenter  Large-scale culture vessel, usually applied to cells in suspension (for animal cells or microorganisms).

Ferritin  Iron-binding protein that stores iron in body.

Fertilization  Fusion of a female and male gamete (both haploid) to form a diploid zygote, which develops into a new individual.

Fetus  Period of mammalian development from second month of intra-uterine life until birth.

Fibroblast  A proliferating precursor cell of the mature differentiated fibrocyte.

Fibroblastic  Resembling fibroblasts, i.e., spindle shaped (bipolar) or stellate (multipolar) ; usually arranged in parallel arrays at confluence if contact inhibited. Often used indiscriminately for undifferentiated mesodermal cells regardless of their relationship to the fibrocyte lineage. Implies a migratory type or cell with processes exceeding the nuclear diameter by threefold or more.

Fibrinolytic system  Cascade of plasma enzymes that breaks down clots; also called thrombolytic system.

Fibronectin  An extracellular multiadhesive protein that binds to other matrix components, fibrin, and cell-surface receptors of the integrin family. It functions to attach cells to the extracellular matrix and is important in wound healing.

Field inversion gel electrophoresis (FIGE)  An electrophoresis technique for the separation of large DNA molecules.

Finite cell line  A culture that has been propagated by subculture but is only capable of a limited number of cell generations in vitro before dying out.

Flagellum (pl. Flagella)  Membrane-enclosed locomotory structure extending from the surface of eukaryotic cells and composed of a specific arrangement of microtubules, called an axoneme. Usually there is only one flagellum per cell (as in sperm cells), and its bending propels the cell forwards or backwards. Bacterial flagella are much simpler structures containing a single predominant type of protein.

Fluorescence activated cells sorter (FACS)  A machine that analyses cells by flow cytometry and then allows them to be sorted into different populations and collected.

Fluorescence in situ hybridization (FISH)  A hybridization technique that uses fluorochromes of different colors to enable two or more genes to be located within a chromosome preparation in a single in situ experiment.<

Follicular dendritic cells  Antigen-presenting cells present in the B-cell areas of lymphoid tissues which retain stores of antigen.

Footprinting  DNase technique for identifying DNA regions that bind protein by digesting a radiolabeled DNA sample with DNase in the presence or absence of a DNA-binding protein and then subjecting the samples to gel electrophoresis. Because regions of DNA with bound protein are protected from digestion, the patterns of fragment bands from protected and unprotected samples will differ, permitting identification of the protein-binding regions of the DNA.

Freund’s adjuvant  An emulsion of aqueous antigen in oil. Complete Freund’s adjuvant contains killed Mycobacterium tuberculosis, while incomplete Freund’s adjuvant does not.

Free energy (G)  A measure of the potential energy of a system that is a function of the enthalpy (H) and entropy (S).

Free-energy change (ΔG)  The difference in the free energy of the product molecules and of the starting molecules in a chemical reaction, which can be used to predict the equilibrium of a reaction. A large negative value of ΔG indicates that a reaction has a strong tendency to occur from left to right, as written

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