Properties of Immunoglobulin M (IgM):

• Is the largest antibody characterized by the µ heavy chain
• Has a molecular weight of 970 kDa
• Is the third most common serum immunoglobulin, constituting 5 – 10% of serum immunoglobulins 
• Exists in 2 forms – a pentamer and a membrane-bound monomer expressed on the surface of B lymphocytes for antigen recognition
• Has a half-life of 5 to 6 days
• Is the first immunoglobulin developed during fetal development at 20 weeks but prevented from crossing the placenta due to its size
• Is synthesized by the neonate and involved in the pathogenesis of some autoimmune diseases