Immunoglobulins (Igs) represent a rather heterogeneous group of proteins with antibody function, synthesized by plasma cells. They are tetrameric glycoproteins consisting of two heavy chains of ~440 amino acids and two light chains of ~220 amino acids. The heavy H (γ, α, μ, δ, ε) and light L (κ, λ) chains are held together by intercatenary disulfide bridges. Each chain is divided into a constant and a variable (amino-terminal) region; on the latter are located the amino acid sequences that contribute to the formation of the antigen- binding site. Some structural differences in the constant region allow further differentiation of Ig classes into sub classes. IgGs (γ2ĸ2/γ2λ2), with a molecular weight of ~150 kDa, are monomers present in the circulation with the highest concentration and are the circulating antibodies of the secondary response; IgAs, (α2ĸ2/α2λ2), with a molecular weight of ~160 kDa, are also monomers; these antibodies are present as dimers on the surface of the mucous mem branes, especially of the bronchial and intestinal tracts. Finally, IgMs (μ2ĸ2/μ2λ2)5 are high-molecular-weight pen tamers (~970 kDa); they are the circulating antibodies of the primary response. The reference values in plasma for adults, according to the IFCC international standardization (CRM 470), are: for IgA 0.7–4.0 g/L; for IgG 7.0–16.0 g/L and for IgM 0.4–2.3 g/L.

The role of Ig measurement in the diagnostic pathway of plasma cell dyscrasias is described in the dedicated para graph. The increase in polyclonal Ig synthesis can be caused by infectious, inflammatory, autoimmune, or neoplastic processes. Increased plasma concentrations can therefore be associated with a wide variety of diseases, but in most cases these are nonspecific responses that add little or nothing to the diagnosis or management of the patient. The most useful measurement of Ig is linked to the diagnostic pathway of immunodeficiencies, within which the measurement of serum Ig is one of the key parameters. The establishment of a suspected immunodeficiency in patients with severe, recurrent, or atypical infections therefore typically begins with the measurement of serum Ig. The most frequent is selective IgA deficiency (approximately 1/400 in Europe), infrequently linked to clinical problems. Measurement of IgA is critical within the diagnostic pathway for celiac disease because the specific antibodies are IgA antibodies; anaphylactic reactions may occur if products containing traces of IgA are administered to patients with total IgA deficiency, as may occur during therapeutic IgG administration, so measurement of IgA in these patients is appropriate. The IgG4 subclass is related to an infrequent specific clinical situation (IgG4-related disease) that requires its measurement in the laboratory.

European guidelines on the management of cholestatic diseases indicate that treatment of primary biliary cirrhosis with ursodeoxycholic acid induces a marked decrease, in addition to liver enzymes, of IgM, one of the typical signs of the disease; the measurement of IgM is therefore indicated in the diagnosis and monitoring of therapy.

مواضيع ذات صلة

Complement

Ceruloplasmin

glucose tolerance test (GTT, Oral glucose tolerance test [OGTT])

glucose, postprandial (2-Hour postprandial glucose [2-Hour PPG], 1-Hour glucose screen)

α1-antitrypsin

Albumin

Diagnostic Laboratory Tests for Mycobacterium Tuberculosis

Glucose, blood (Blood sugar, Fasting blood sugar [FBS])

Gliadin, endomysial, and tissue transglutaminase antibodies

Antimicrobial Susceptibility Testing and Therapy of Staphylococcus, Micrococcus, and Similar Organisms

Glucagon

genetic testing (Breast cancer [BRCA] and ovarian cancer, Colon cancer, Cardiovascular disease, Tay–Sachs disease, Cystic fibrosis, Melanoma, Hemochromatosis, Thyroid cancer, Paternity [parentage analysis], Forensic genetic testing)