IMMUNOLOGY

Detailed explanation-1: -Natural immunoglobulin M (IgM) antibodies are pentameric or hexameric macro-immunoglobulins and have been highly conserved during evolution. IgMs are initially expressed during B cell ontogeny and are the first antibodies secreted following exposure to foreign antigens.

Detailed explanation-2: -Pentameric IgM is stabilized by covalent, disulfide bonds between the Fc regions of IgM molecules, as well as disulfide bonds between the J chain and the Fc regions of IgM. Hexameric IgM in humans is less well characterized than pentameric IgM, and hexameric IgM assembles independently of the J chain.

Detailed explanation-3: -Soluble immunoglobulin M (IgM) forms a pentamer containing a joining (J) chain polypeptide.

Detailed explanation-4: -IgM is the first class of immunoglobulin made by B cells as they mature, and it is the form most commonly present as the antigen receptor on the B-cell surface. When IgM is secreted from the cells, five of the basic Y-shaped units become joined together to make a large pentamer molecule with 10 antigen-binding sites.

Detailed explanation-5: -(B) Pentameric IgM contains five monomers and one J chain; disulfide bonds between each monomer form the pentamer; the structure shown in (B) is based upon the recent EM image presented by Hiramoto et al.