farmer’s lung A hypersensitivity disease caused by the interaction of IgG antibodies with large amounts of an inhaled antigen in the alveolar wall of the lung, causing alveolar wall inﬂammation and compromising respiratory gas exchange.
Fc fragment, Fc region The carboxy-terminal halves of the two heavy chains of an IgG molecule disulfide-bonded to each other by the residual hinge region. It is produced by cleavage of IgG by papain. In the complete antibody this portion is often called the Fc region.
Fc receptors Family of cell-surface receptors that bind the Fc portions of different immunoglobulins: Fcγ receptors bind IgG, for example, and Fcε receptors bind IgE.
FCAS See familial cold autoinflammatory syndrome.
FcεRI The high affinity receptor for the Fc region of IgE. Expressed primarily on the surface of mast cells and basophils. When multivalent antigen interacts with IgE that is bound to FcεRI and cross-links nearby receptors, it causes activation of the receptor-bearing cell.
FcγR1 (CD64) Fc receptor highly expressed by monocytes and macrophages that has the highest affinity of the Fc receptors for IgG.
FcγRIIB-1 An inhibitory receptor on B cells that recognizes the Fc portion of IgG antibodies. FcγRIIB-1 contains an ITIM motif in its cytoplasmic tail.
FcγRIII Cell-surface receptors that bind the Fc portion of IgG molecules. Most Fcγ receptors bind only aggregated IgG, allowing them to discriminate bound antibody from free IgG. Expressed variously on phagocytes, B lymphocytes, NK cells, and follicular dendritic cells, the Fcγ receptors have a key role in humoral immunity, linking antibody binding to effector cell functions.
FcRn (neonatal Fc receptor)   Neonatal Fc receptor, a receptor that transports IgG from mother to fetus across the placenta, and across other epithelia such as the epithelium of the gut.
FHL See familial hemophagocytic lymphohistiocytosis.
fibrinogen-related proteins (FREPs) Members of the immunoglobulin superfamily that are thought to have a role in innate immunity in the freshwater snail Biomphalaria glabrata.
ficolins Carbohydrate-binding proteins that can initiate the lectin pathway of complement activation. They are members of the collectin family and bind to the N-acetylglucosamine present on the surface of some pathogens.
fingolimod Small-molecule immunosuppressive drug that interferes with the actions of sphingosine, leading to retention of effector T cells in lymphoid organs.
FK506 See tacrolimus.
FK-binding proteins (FKBPs) Group of prolyl isomerases related to the cyclophilins and bind the immunosuppressive drug FK506 (tacrolimus).
flagellin A protein that is the major constituent of the ﬂagellum, the tail-like structure used in bacterial locomotion. TLR-5 recognizes intact ﬂagellin protein that has dissociated from the ﬂagellum.
fluid-phase C3 convertase Short-lived alternative pathway C3 convertase, C3(H20)Bb, that is continually produced at a low level in the plasma that can initiate activation of the alternative pathway of complement.
fMet-Leu-Phe (fMLF) receptor A pattern recognition receptor for the peptide fMet-Leu-Phe, which is specific to bacteria, on neutrophils and macrophages. fMet-Leu-Phe acts as a chemoattractant.