Unit Code 200518:
IgG Subclasses, Serum
Useful For
Evaluating patients with clinical signs and symptoms of
humoral immunodeficiency or combined immunodeficiency (cellular
and humoral)
Testing for IgG subclass levels may be indicated in
hypogammaglobulinemic patients, and in patients with clinical evidence
of a possible immunodeficiency with a normal level of total IgG protein
in serum.
Clinical Information
The most abundant immunoglobulin in human serum is IgG
(approximately 80% of the total). IgG protein is comprised of
molecules of 4 subclasses designated IgG1 through IgG4.
Each subclass contains molecules with a structurally unique gamma
heavy chain. Of total IgG, approximately 65% is IgG1, 25% is IgG2,
6% is IgG3, and 4% is IgG4. Molecules of different IgG subclasses
have somewhat different biologic properties (eg, complement
fixing ability and binding to phagocytic cells), which are determined
by structural differences in gamma heavy chains.
Diminished concentrations of IgG subclass proteins may occur in
the context of hypogammaglobulinemia (eg, in common variable
immunodeficiency where all immunoglobulin classes are generally
affected) or deficiencies may be selective, usually involving IgG
subclass 2. Deficiency of IgG1 usually occurs in patients
with severe immunoglobulin deficiency involving other IgG subclasses.
Deficiency of IgG2 is more heterogeneous and can occur
as an isolated deficiency or in combination with deficiency of IgA,
or of IgA and other IgG subclasses. Most patients with IgG2 deficiency
present with recurrent infections, usually sinusitis, otitis, or pulmonary
infections. Children with deficiency of IgG2 often have
deficient antibody responses to polysaccharide antigens including
bacterial antigens associated with Haemophilus influenzae type B
and Streptococcus pneumoniae. Isolated deficiencies of IgG3 or IgG4
occur rarely, and the clinical significance of these findings is
not clear.
Reference Values
See "IgG Subclasses Reference Values" in Special Instructions.
TOTAL IgG
0-4 months: 141-930 mg/dL
5-8 months: 250-1,190 mg/dL
9-11 months: 320-1,250 mg/dL
1-3 years: 400-1,250 mg/dL
4-6 years: 560-1,307 mg/dL
7-9 years: 598-1,379 mg/dL
10-12 years: 638-1,453 mg/dL
13-15 years: 680-1,531 mg/dL
16-17 years: 724-1,611 mg/dL
> or = 18 years: 600-1,500 mg/dL
Interpretation
Diminished concentrations of all IgG subclasses are found in common
variable immunodeficiency, combined immunodeficiency, ataxia
telangiectasia, and other primary and acquired immunodeficiency
diseases.
A diminished concentration of IgG2 protein may be clinically significant
in the context of recurrent sinopulmonary infection and may occur with
or without concomitant IgA deficiency.
Slightly diminished concentrations of 1 or more IgG subclass proteins
is not uncommon, and usually have little clinical significance. Conversely,
some individuals with deficient specific antibody responses to polysaccharide
antigens may have normal serum levels of IgG subclasses.
Cautions
Measurement of IgG subclass proteins is not a first-order test in
patients suspected of having an immunodeficiency disease.
Quantitation of IgG, IgA, and IgM levels, along with other first-order
tests for immunodeficiency, should be performed first.
Special Instructions and Forms
Clinical Reference
Umetsu DT, Ambrosino DM, Quinti I, et al: Recurrent sinopulmonary
infection and impaired antibody response to bacterial capsular
polysaccharide antigen in children with selective IgG-subclass
deficiency. N Engl J Med 1985;313:1247-1251
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