MRSA Support Group

— Selective deficiency of immunoglobulin M (sIgM-D) is an immune disorder that has been reported in association with serious infections, such as bacteremia. Cumulatively, there have been fewer than 200 to 300 cases reported in the literature, and understanding of this condition is therefore preliminary. The observations and recommendations in this review are based largely upon case reports and small series.

SIgM-D is characterized by the following clinical and laboratory features:

Isolated absence or deficiency of IgM
Normal levels of other immunoglobulins (notably IgG and IgA)
Normal T cell numbers
No other identifiable immunodeficiency
Recurrent infections from infancy onwards, associated with susceptibility to Staphylococcus aureus, encapsulated pathogens (Streptococcus pneumoniae, Hemophilus influenza), and viral infections
An association with autoimmune and malignant conditions
The following discussion reviews the immunophysiology of IgM, possible etiologies for selective IgM deficiency, the clinical manifestations of the disorder, as well as an approach to its diagnosis and management.

NORMAL IMMUNOPHYSIOLOGY OF IGM
— The five classes or isotypes of immunoglobulins that contribute to humoral immunity are immunoglobulin (Ig) G, IgM, IgA, IgD, and IgE. (See "Structure of immunoglobulins" and see "Function and clinical applications of immunoglobulins").

Naive B lymphocytes express both IgM and IgD, and these are the only immunoglobulin isotypes produced by B lymphocytes prior to contact with antigen. Membrane bound monomeric IgM, complexed with additional proteins such as Ig alpha and Ig beta, constitute the B cell receptor for antigen. Naive B lymphocytes use alternative mRNA splicing to make both IgM and IgD. The mu and delta constant region (C) genes are those nearest to the assembled VDJ sequence
Posted on 08/04/09, 09:08 am