The RNA transcript of C-terminus of the heavy chains of secreted form is alternatively spliced during RNA processing. More specifically in IgM at the first site (pAμs), eliminating the MC exons and retaining the polyA tail giving rise to the secreted form of the heavy chain with shorter C end.
IgM is the largest antibody, and it is the first antibody to appear in the response to initial exposure to an antigen. IgM is constructed of five or six units (i.e. mostly as pentamers but also hexamers occur) which are each comprised of two heavy-chains (mu-chains) and two light chains, bound together by disulfide bonds and a so-called J-chain