IgM

Monomeric IgM

IgM only exists as a monomer on the surface of 

B cells.

Monomeric IgM has a very low affinity for antigen

Cm4 contains the transmembrane and cytoplasmic 

regions. 

These are removed by RNA splicing to produce 

secreted IgM

Polymeric IgM

IgM forms pentamers and hexamers.

Cm3 binds C1q to initiate activation of the classical 

complement pathway

Cm1 binds C3b  to facilitate uptake of opsonised 

antigens by macrophages

Cm4 mediates multimerisation 

(Cm3 may also be involved)