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Igα/Igβ (CD79a/CD79b) 352 IgG subclass deficiency
immunoreceptor tyrosine-based activation motifs (ITAMs)
that participate in early signaling when antigens activate B
cells.
Ig/Ig (CD79a/CD79b)
The Igα/Igβ heterodimer interacts with immunoglobulin
heavy chains for signal transduction. In the pro-B cell stage,
rearrangement of the immunoglobulin heavy chain gene
leads to expression of surface membrane immunoglobulin
(mIgμ). mIgμ associates with Igα/Igβ and surrogate light
chain in pre-B cells or ordinary light chains in B cells to
form the precursor B cell receptor and B cell receptor,
respectively. Igα and Igβ are expressed before immunoglobulin
heavy chain gene rearrangement. They are products
of mb-1 and B29 genes, respectively. Allelic exclusion is
mediated through signal transduction via Igα and Igβ and
depends on intact tyrosine residues.
IgA
Refer to immunoglobulin A.
IgA deficiency
Refer to immunoglobulin A deficiency.
IgA nephropathy. Granular immune deposits in mesangial areas.
IgA nephropathy (Berger’s disease). Granular immune deposits containing
IgG, IgA, and C3 in mesangial areas.
IgA nephropathy (Berger’s disease)
A type of glomerulonephritis in which prominent IgAcontaining
immune deposits are present in mesangial areas.
Patients usually present with gross or microscopic hematuria
and often mild proteinuria. Mesangial widening or proliferation
may be observed by light microscopy; however,
immunofluorescence microscopy demonstrating IgA and C3
fixed by the alternative pathway is requisite for diagnosis.
Electron microscopy confirms the presence of electrondense
deposits in mesangial areas. Half of the cases progress
to chronic renal failure over a 20-year course.
IgA paraproteinemia
Myeloma in which the paraimmunoglobulin belonging
to the IgA class occurs in only about one fifth of affected
patients. IgA myeloma patients are reputed to have lower
survival rates than those patients affected with IgG
myeloma, but this claim awaits confirmation. IgA myeloma
may be associated with myeloma cell infiltration of the liver
leading to jaundice and altered liver function tests. It is
frequently associated with hyperviscosity syndrome, which
may be related to the ability of IgA molecules to polymerize
and form complexes with such substances as haptoglobin,
α-1 antitrypsin, β lipoprotein, antihemophilic factor, and
albumin.
IgD
Refer to immunoglobulin D.
IgE
Refer to immunoglobulin E.
IgG
Refer to immunoglobulin G.
IgG index
The ratio of IgG and albumin synthesis in the brain and in
peripheral tissues. It is increased in multiple central nervous
system infections, inflammatory disorders, and neoplasms.
IgG-induced autoimmune hemolysis
Two fifths of cases of this hemolysis are secondary to other
diseases such as neoplasia including chronic lymphocytic
leukemia and ovarian tumors. This hemolysis may also
be secondary to connective tissue diseases such as lupus
erythematosus, rheumatoid arthritis, and progressive
systemic sclerosis. Patients experience hemolysis leading
to anemia with fatigue, dizziness, palpitations, exertion
dyspnea, mild jaundice, and splenomegaly. They manifest
positive Coombs’ antiglobulin tests. Their erythrocytes
appear as spherocytes and schistocytes, and evidence of
erythrophagocytosis is present. Erythroid hyperplasia of
the bone marrow and lymphoproliferative disease may be
present. Glucocorticoids and blood transfusions are used
for treatment. Approximately 66% improve after splenectomy
but often relapse. Three quarters of these individuals
survive for a decade.
IgG myeloma subclasses
The most frequent myeloma paraimmunoglobulin is IgG-κ,
which belongs to the IgG 1 subclass most commonly and to
the IgG 4 subclass only rarely. IgG 3 paraimmunoglobulins
are associated with increased serum viscosity, which leads
to aggregates that are related to concentration and may be
associated with serum cryoglobulin. In IgG paraimmunoglobulin
myeloma, the serum concentration of IgG may
reach 2000 mg/dL. Concomitantly there is a reduction in
other immunoglobulins. Because the IgG myeloma paraimmunoglobulin
belongs to one heavy-chain subtype, the Gm
allotypes are restricted in a manner similar to the restriction
of light chains to either the κ or λ type.
IgG subclass deficiency
Decreased or absent IgG 2, IgG 3, or IgG 4 subclasses. Total
serum IgG is unaffected because it is 65 to 70% IgG 1.
Deletion of constant heavy chain genes or defects in isotype
switching may lead to IgG subclass deficiency. IgG 1 and
IgG 3 subclasses mature faster than IgG 2 or IgG 4. Patients