It normally exists as a pen tamer in serum but can also occur as a monomer. It has an extra domain on the mui chain (CH4) and another protein covalently bound via S-S . called J-chain. This chain helps it to polymerize to the pentamer form.
Properties
1. It is the first Ig to be made by fetus in most species and new B cells when stimulated by Ags.
2. It is the 3rd most abundant Ig in serum.
3. It is a good complement fixing Ig leading to lyses of microorganisms
4. It is also a good agglutinating Ig, hence clumping microorganisms for eventual elimination from
the body.It is also able to bind some cells via Fc receptors.B cells have surface IgMs , which exists as monomers and lacks J chain but have an extra 20 amino acid at the C-terminal that anchors it to the cell membrane.