Immunoglobulin M (IgM) normally constitutes about 10% of serum immunoglobulins. IgM antibody is prominent in early immune responses to most antigens and is largely confined to plasma due to it's large size. Monomeric IgM is expressed as a membrane bound antibody on the surface of B cells and as a pentamer when secreted by plasma cells. Due to it's high valency IgM is more efficient than other isotypes is binding antigens with repeating epitopes (virus particles and red blood cells) and is more efficient than IgG in activiating the complement pathway. The gene for the mu constant region contains four domains separated by short intervening sequences.