IgM (Macroglobulin)

IgM (MACROGLOBULIN)

  • It is also secreted by plasma cells in spleen, bone marrow and lymph nodes.
  • It has sedimentation co-efficient of 19S (900 KDa) and constitutes about 5-8% of all immunoglobulins and giving rise to second highest concentration in mammalian serum.

Structure

  • On B cell surface, IgM is single monomer (180KDa) but the secreted IgM is a pentamer (occasionally hexamer).
  • Five monomers of IgM are linked in a circular fashion by disulphide bonds between heavy chins and one-addition chain rich in cysteine and asparagine called J Chain (15 KDa) joing two monomers to complete the circle.
  •  J chain is about 118-125 amino acid long.
  • Each IgM monomer has two heavy μ chains and 2 light chains of either κ or λ.
  • Each heavy chain has an additional domain (CH4) at the C terminus of FC region.
  • IgM do not have hinge region and contain about 12 % Carbohydrate.
  • IgM is predominantly present intavascularly (80%) and theoretically the pentamer IgM has valence of 10 (each subunit posses two antigens binding sites) but practically found to be 5 probably due in steric hindrance.
  • IgM found in serum (pentamer) can be disrupted by treating with 2-mercaptoethanol (0.12M) into monomers, as disulphide bounds joining the monomers are broken. This treatment differentiates IgM antibodies from IgG antibodies.
  • IgM is the first antibody produced in primary response to antigen and half-life is relatively shorter than IgG. Based on peptide mapping and complement fixing activity IgM is classified into IgM1 and IgM2 in humans but not in other animal species. IgM has high avidity.

IgM

 

Functions

  • Biologically more active than IgG. A single molecule of IgM can cause immune hemolysis where as 1,000 IgG are required for the same effect.
  • IgM is 500-1000 times more effective than IgG in opsonization, 100 times more effective in bactericidal action and 20 times more effective in agglutination.
  • The complement-binding site is present in CH4 and CH3 domains. It acts as complement fixing antibody. It is more active than IgG.
  • It is the first antibody produced in primary immune response.
Last modified: Friday, 23 September 2011, 10:40 AM