Immunoglobulin M

INTRODUCTION:
  • Normal serum level: 0.5 - 2 mg/ml
  • Called Millionaire molecule (19S molecule - heavy molecular weight)
  • Pentameric structure
  • Polymerization of subunits depend upon the presence of J chain
  • Mostly intravascular
  • First to appear following infection and disappear early; hence their demonstration in serum indicates recent infection - primary immune response
  • Earliest Ig synthesized by fetus
  • Does not cross placenta and hence its detection is useful in IU and congenital infections
  • More effective than IgG in opsonisation, bacterial agglutination and bactericidal activity
  • Treatment of serum with 2-mercaptoethanol selectively destroys IgM without affecting IgG
  • Examples of IgM: isohemagglutinins (anti-A, anti-B), antibodies to typhoid 0 antigen, reagin antibodies in syphilis
STRUCTURE:
  • Pentamer , so it can bind 10 antigen at a time.
  • Extra domain (CH4)
  • Also contain J chain which form disulfid bond with hinge region
  • IgM molecules are polymers of five four-peptide subunits
  • Diameter 300 Angstrom
  • Length of monomer is 100 A
  • 25-30 A width.
PROPERTIES:
  • 3rd highest serum Ig
  • First Ig made by fetus and B cells
  • Fc piece of IgM Fixes complement
  • IgM is responsible for opsonisation
  • Can not cross placenta
  • Complete antibodies
  • Usual temp 20c but best temperature is 4c
  • Agglutinating antibody
CONDITIONS AFFECTING IgM:
IgM increases (in adults) in:

  • Wsaldenstrom's macroglobulinemia
  • Trypanosomiasis
  • Actinomycosis
  • Carrion's disease (bartonellosis)
  • Malaria
  • Infectious mononucleosis
  • Lupus erythematosus
  • Rheumatoid arthritis
  • Dysgammaglobulinemia (certain cases)
Note: In the newborn, a level of IgM above 20 ng./ dl is an indication of in utero stimulation of the immune system and stimulation by the rubella virus, the cytomegalovirus, syphilis, or toxoplasmosis.

IgM decreases in:
  • Agammaglobulinemia
  • Lymphoproliferative disorders (certain cases)
  • Lymphoid aplasia
  • IgG and IgA myeloma
  • Dysgammaglobulinemia
  • Chronic lymphoblastic leukemia
Exam Question
  • IgM is a pentamer
  • IgM is produced by in-utero infection
  • The earliest immunoglobulin to be synthesized by the fetus is IgM
  • IgM contains J chain in it's structure
  • IgM is the first immunoglobulin to appear following infection
  • IgM is responsible for opsonisation
  • IgM fixes complement
  • IgM increased in primary response
  • IgM molecules are polymers of five four-peptide subunits

Don't Forget to Solve all the previous Year Question asked on Immunoglobulin M