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Fcγ R 269 feline immunity
Fc R
The three principal R types of FcRγ include Fcγ RI, Fcγ
RII, and Fcγ RIII that differ in their ITAMs/ITIMs content,
which determines their role in effector cell activation and
in their IgG affinity. Fcγ R I (CD64), the high affinity IgG
receptor, is expressed exclusively on activated granulocytes
and is upregulated by inflammatory cytokines. Fcγ
R I is comprised of an α chain that binds IgG and and
FcRγ homodimer that contains ITAMs. Fcγ R I unites with
the C terminal region of a single H chain comprising the
Fc region of an IgG molecule. It has the greatest affinity
for IgG1 or IgG3 antibodies. Thus, these IgG, subtypes
are excellent opsonins. Fcγ R I interacts with IgG, leading
to inflammatory cytokine secretion and macrophage
and monocyte induced target cell lysis through an ADCC
mechanism. It also results in the generation of superoxide
by neutrophils. Fcγ RII (CD32) has broader expression but
lower affinity for noncomplexed IgG molecules than does
Fcγ RI. Fcγ RII reacts only with aggregated IgG that is
bound to a multivalent antigen. It is comprised of a single
transmembrane chain. Phagocytes, as well as megakaryocytes
and platelets, express Fcγ RIIA, which contains
ITAM. IgG union with Fcγ RIIA leads to a respiratory burst
in neutrophils and causes platelets to secrete inflammatory
molecules. Fcγ RIIB contains ITIM rather than an ITAM
sequence. It is expressed by lymphoid and myeloid cells.
It is believed that this receptor in B cells may signal the
cell to decrease antibody synthesis when a critical level of
antibody has become bound to surface Fcγ R IIB molecules.
Macrophages, monocytes, neutrophils, megakaryocytes
and platelets express Fcγ RIIC receptors, which possess
an ITAM and show only low binding affinity for IgG. The
Fcγ RIII receptor (CD16) manifests low IgG affinity. The
Fcγ RIIIA isoform is comprised of an IgG binding α chain
and an ITAM-containing FcRγ chain dimer. Phagocytes
and NK cells are the principal sites of Fcγ RIIIA expression.
This isoform may be found also on B cells, mast cells,
Langerhans’ cells, and γδ T cells. The only FcR found on
NK cells is Fcγ RIIIA, which has a classic FcR transmembrane
region that facilitates NK cell induced ADCC and
degranulation of mast cells. Neutrophils alone express
the Fcγ RIIIB isoform, which is comprised of a single-
chain protein without transmembrane chains or ITAMs.
Phosphatidyl inositol residues fix Fcγ RIIIB in the membrane.
Neutrophils employ Fcγ R III receptors to convey a
requirement for vigorous phagocytosis.
Fc RI
Represents a high affinity receptor found on mononuclear
phagocytes. In humans it binds IgG1 and IgG3.
Fc RII
A membrane protein designated as CD32 that serves as
a receptor for the Fc regions of immunoglobulin G (IgG)
molecules.
Fc RIII
Fcγ RII and Fcγ RIII represent low affinity IgG receptors.
In humans, neutrophils, monocytes, eosinophils, platelets,
and B lymphocytes express Fcγ RII on their membranes.
Neutrophils, natural killer cells, eosinophils, macrophages,
and selected T lymphocytes express Fcγ RIII on their
membranes and bind IgGl and IgG3. Paroxysmal nocturnal
hemoglobulinuria patients have deficient Fcγ RIII on their
neutrophil membranes.
FcRn (neonatal)
Fc receptors expressed by neonatal intestinal cells that
bind IgG in breast milk. They facilitate its passage into the
neonatal intestinal lumen and also aid IgG transport in the
adult intestine and placenta. Also called Brambell receptor.
FDCs
Refer to follicular dendritic cells.
Fd fragment
The heavy chain portion of a Fab fragment produced by
papain digestion of an immunoglobulin G (IgG) molecule.
It is on the N terminal side of the papain digestion site.
Fd piece
Refer to Fd fragment.
Fd′ fragment
The heavy chain portion of an Fab′ fragment produced by
reduction of the F(ab′) 2 fragment that results from pepsin
digestion of IgG. It is comprised of V H1, C H1, and the heavy
chain hinge region. Fd contains 235 amino acid residues.
Fd′ piece
Refer to Fd′ fragment.
FDNB (1-fluoro-2-4-dinitrobenzene)
Refer to dinitrofluorobenzene (DNFB).
Feline immunity.
feline immunity
Although peripheral lymphoid tissues and thymus glands of
cats are comparable to those of other mammals, they possess
a population of pulmonary intravascular macrophages that
may cause the animals to manifest increased susceptibility
to septic shock mediated by macrophage-derived tumor
necrosis factor. Forty to 45% of feline peripheral blood
lymphocytes are B cells, and 32 to 41% are T cells; 20% of
feline peripheral blood cells are null cells considered to be
natural killer (NK) cells. Both T helper and T suppressor
activities have been described. Feline interleukin-1 (IL1)
and IL2 as well as IL6 have been identified, although the
physical and chemical properties of the latter are different
from those of the human and mouse interleukins.
Interferons α, β, and γ have been characterized and resemble
those of other species. Although all immunoglobulin
isotypes of other species are recognized in cats, neither IgE
nor IgD has been formally identified. Cats also have secretory
IgA with a J-chain that resembles equivalent molecules
of other species. Although cats do not respond to tuberculin,
they develop good delayed hypersensitivity responses
(viral antigens) in the skin to dinitrochlorobenzene and
F