Immunoglobulin (Ig)

Characteristics

All Igs consists of 2 heavy chains and 2 light chains. The heavy chains define the Ig class as Ig A (alfa), IgG (gamma) IgM (my), IgD (delta) and IgE (epsilon). The light chains are designated kappa and lambda. Whatever type of heavy chain each Ig-molecule consists of either 2 kappa or 2 lambda light chains. Igs are produced in mature B-lymphocytes from from the virgin small B-lymphocyte through its activated stages until the plasma cell stage, but the amount of Ig in different B-lymphocytes vary considerably. Virgin unstimulated B-lymphocytes contain only small amounts of cytoplasmic my-chains. During maturation B-lymfocytes in the mantle zone express surface IgM (Fig. 1) and IgD. When B-cells are activated and transform into plasma cells, they become capable of cytoplasmic accumulation and secretion of Igs, while the membranous expression is lost. Most activated B-cells produce IgM but about 10% accumulate and secrete another Ig-type, mostly IgG or IgA. Ig-molecules containing kappa light chains are slightly more frequent than molecules containing lambda.

Neoplasms

While benign (reactive) B-lymphocytic populations produce Ig-molecules containing an almost equal amount of kappa and lambda light chains, i.e. the number of cells producing kappa is more or less equal to the number producing lambda, neoplastic B-lymphocytic populations has light chain restriction (i.e., are monoclonal) producing either kappa or lambda. Consequently, demonstration of light chains is the most important procedure in the diagnosis of neoplasms of B-lymphocytes (lymphomas and leukemias) (Fig 2).

The amount of Ig produced in individual types of B-lymphocytic neoplasia vary and is often small, demanding a sensitive technique for detection.

About 80% of non-Hodgkin lymphomas are B-cell lymphomas, of which the large majority express surface (s) IgM, although the expression in small cell lymphoma/chronic lymphocytic leukaemia is weak. Plasmacytoma/multiple myeloma do not show sIgM. Lymphoplasmacytic lymphoma most often shows sIgM, while plasmacytoma/multiple myeloma show sIgG in 50% and sIgA in 20%.

Application

Light chain restriction is the single most important marker for neoplasms of B-lymphocytic origin. Demonstration of heavy chains be sometime be of aid in the study of malignant lymphomas, as lymphoplasmacytic lymphomas are usually focally IgM positive, while plasmacytoma/multiple myeloma in most cases express strong cytoplasmic IgG or IgA. Precursor B-cell neoplasms are are Ig negative.
 

Visualization

For formalin fixed, paraffin embedded material enzyme digestion improve sensitivity but HIER methods are more sensitive and should be the method of choice. Detection of Ig in tissue sections is often hampered by the presence of serum and thereby Ig in the tissue causing background staining, which sometimes makes the detection of small amounts of Ig on cell surfaces or in the cytoplasm impossible. Poorly fixed and necrotic tissue showing non-specific binding of Ig-antibodies may also preclude detection of clonality. Immunoglobulin taken up by histiocytes will be detected even though the cells are not of lymphocytic origin.
Control tissue: Tonsil. Plasma cells should reveal a strong cytoplasmic staining. Approximately 50% of the mantle zone B-cells should show a distinct membranous staining reaction for light chain kappa or lambda, while the rest should be unstained. All mantle zone B-cells should display a strong IgM membranous staining reaction.

Assessments

IgK Run 15 2005

IgL Run 15 2005

IgK Run 18 2006

IgM Run 18 2006
IgK Run 23 2008
IgM Run 23 2008
IgM Run 30 2010

Selected references

Jaffe ES et al (Eds) WHO Classification Tumours of Haematopoietic and Lymphoid Tissue. IARC Press 2001.

Ashton-Key M, Jessup E, Isaacson PG. Immunoglobulin light chain staining in paraffin-embedded tissue using a heat mediated epitope retrieval method. Histopathology. 1996 Dec;29(6):525-31.