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Fc region 268 Fcγ receptors
effect requires prior binding of their Fc–IgG receptors by
IgG immune complexes. T M and T G cells differ in a number
of ways. Circulating T G cells may be present in increased
numbers, often accompanied by a reduction in the circulating
number of T M cells. Increased numbers of T G cells are seen
in cord blood and in some patients with hypogammaglobulinemia,
sex-linked agammaglobulinemia, IgA deficiency,
Hodgkin disease, and thymoma, to mention only a few.
Fc region
Refer to Fc fragment.
Fc′ fragment
A product of papain digestion of immunoglobulin G (IgG). It
is comprised of two noncovalently bonded C H3 domains that
lack the terminal 13 amino acids. This 24-kDa dimer consists
of the region between the heavy-chain amino acid residues
14 through 105 from the carboxyl terminal end. Normal
human urine contains minute quantities of Fc′ fragment.
FcεRI.
Fcε receptor (FcR)
Mast cell and leukocyte high affinity receptor for the Fc
region of immunoglobulin E (IgE). When immune complexes
bind to Fcε receptors, cells may respond by releasing the
mediators of immediate hypersensitivity such as histamine
and serotonin. Modulation of antibody synthesis may also
occur. The two varieties of Fcε receptors are designated
FcεRI and FcεRII (CD23). FcεRI represents a high affinity
receptor found on mast cells and basophils, anchors
monomeric IgE to the cell surface, and possesses 1α, 1β, and
2γ chains. FcεRII represents a low affinity receptor found
on mononuclear phagocytes, B lymphocytes, eosinophils,
and platelets. Subjects with increased IgE in the serum
have elevated numbers of FcεRII on their cells. FcεRII is
a 321-amino-acid, single polypeptide chain that is homologous
with asialoglycoprotein receptor. FcεRI and FcεRII
are the two subtypes. Blood basophils and tissue mast cells
are the principal sites of FcεRI, which is the high affinity
receptor. Allergic individuals also express it on Langerhans’
cells, eosinophils, and monocytes. Its tetrameric molecular
structure is composed of α and β polypeptides and a dimer
of the FcRγ chain and αβ(γ)2 molecule that contains ITAM.
There is also an ITAM in the cytoplasmic domain of the β
chain. FcεRI binds to monomeric IgE. The interaction of
multivalent antigen with multiple IgE molecules on the mast
cells surface leads to aggregation of the FcεRI configurations
causing degranulation of mast cells, releasing vasoactive
amines that produce local inflammation and allergy
symptoms. FcεRII (CD23) is comprised of one transmembrane
chain and demonstrates less affinity for IgE than does
FcεRI. Its lectin-binding domain facilitates its interaction
with selected carbohydrates on cells. FcεRII is found on
monocytes, eosinophils, B and T cells, but not mast cells or
basophils. The FcεRIIA isoform is found mainly on B cells,
although its function is unknown. It is believed to bind to the
B cell co-receptor molecule CD19. The FcεRIIB molecule is
found on monocytes and eosinophils, which enables them to
fatally injure target cells coated with IgE through an ADCC
mechanism. This isoform induces NO and IL-10 synthesis by
monocytes and macrophages.
FcεRIII.
Fc receptor (FcRI)
A mast cell, basophil and activated eosinophil surface receptor
that combines with free IgE with high affinity. Specific
antigen interaction with the cell-bound IgE molecules leading
to crosslinkage results in cell activation and degranulation.
Fc receptor (FcR)
Receptor for the Fc region of IgG. B and T lymphocytes,
natural killer cells, polymorphonuclear leukocytes, mononuclear
phagocytes, and platelets contain FcγR. When
these receptors bind immune complexes, the cells may
produce leukotrienes and prostaglandins, modulate antibody
synthesis, increase consumption of oxygen, activate
oxygen metabolites, and become phagocytic. The three
types of Fcγ receptors include FcγRI, FcγRII (CD32), and
Fc RIII (CD16). FcγRI represents a high affinity receptor
found on mononuclear phagocytes; in humans, it binds IgG 1
and IgG 3. FcγRII and FcγRIII represent low affinity IgG
receptors. Neutrophils, monocytes, eosinophils, platelets,
and B lymphocytes express FcγRII on their membranes.
Neutrophils, natural killer cells, eosinophils, macrophages,
and selected T lymphocytes express FcγRIII on their
membranes in humans and bind IgG 1 and IgG 3. Patients
with paroxysmal nocturnal hemoglobulinuria have deficient
FcγRIII on their neutrophil membranes.
Fc receptors
Cell surface structures that are specific for IgG antibody Fc
regions. These receptors are different for subclasses of IgG.