IgM

Summary

mu is the heavy chain element of IgM, and may be detected in the cytoplasm of early B cells. Membrane mu is present on mature B cells but following antigen encounter mu is generally replaced by other heavy chain types. Mu expression is used in the immunophenotypic sub-classification of ALL since cytoplasmic mu is acquired by pre-B cells then expressed on the cell surface of mature B-ALL (Burkitt lymphoma) together with surface light chain. Detection of either surface mu or light chain can therefore contribute to the identification of Bukitt lymphoma. The detection of cytoplasmic mu has value in the identification of pre-B ALL subtypes but can be done on bone marow trephine immunostaining.

Background

Mu heavy chain together with kappa or lambda light chains forms the IgM immunoglobulin molecule. In mature b-cells that have not encountered IgM is the predominant immunoglobulin type that forms part of the B-cell receptor (BCR). During B cell development the immunoglobulin heavy chain mu appears first in the cytoplasm where it is associated with surrogate light chains (pre-b cell stage) then subsequently is expressed on the membrane combined with either kappa or lambda light chain in mature B cells. Following antigen encounter IgM is most frequently replaced by other heavy chain forms.

Diagnostic Use

The changing expression of the mu heavy chain has value in the classification of B-lineage ALL

• cytoplasmic mu distinguishes pre-B ALL (mu expressed) from the more common early pre-B ALL (mu not expressed)
• surface expression of mu is a feature of mature forms of ALL including Burkitt lymphoma (where surface light chains will also be detected)
• In histopathology mu expression is used to establish B cell lymphoma type, particularly lymphoplasmacytoid lymphoma where strong cytoplasmic expression is expected.

---