A virgin B-cell bears IgM (and possibly IgD) in its membrane; following stimulation it begins to secrete IgM into its surrounding environment. The difference between the two forms of IgM is entirely at the C-terminus. The ms (secreted) chain has a hydrophilic peptide additional to the Cm4 domain and the mm (membrane-bound) chain has a hydrophobic peptide replacing the former which anchors the molecule into the membrane. The membrane-bound form of IgM is also incapable of associating with J-chain and forming its normal pentameric structure; membrane IgM, therefore, exists exclusively in monomeric form (H2L2), also known as IgMS or "sub-unit" IgM. Difference arises because the ms and mm chains are translated from distinct mRNA molecules that are generated by alternative splicing and/or termination points of transcripts from a single rearranged m gene. Similar differences exist for secreted versus membrane-bound forms of IgG, IgA and IgE heavy chains. The immunoglobulin gene encodes both membrane-associated and secreted proteins through alternative RNA processing reactions. The regulatory mechanisms governing alternative RNA processing involves gene coding for competing cleavage-polyadenylation and RNA splicing reactions and the relative use of the two pathways is differentially regulated between B cells and plasma cells. General cleavage-polyadenylation and RNA splicing reactions are both altered during B cell maturation to affect immunoglobulin expression. Not the others options due to following points... Each B cell is known to express a unique combination of immunoglobulin chains, where each V-J and V-D-J recombination is different. A single productive rearrangement of each type occurs in a given lymphocyte to produce one light and one heavy chain gene. Because each event involves the expression of genes of only one homologous chromosome, the alleles on the other homologous chromosome is not expressed in this same cell. This is called allelic exclusion. Assembly of the light and heavy chains during reaarangement of the variable region of immunoglobulins involves the mechanism indicated by the presence of conserved sequences at the boundaries of all germ line segments that paicipate in joining reactions. Each conserved sequence consist of a palindromic hepatamer called recombination signal sequences (RSS). These act as facilitators of recombination process. These sequences correspond to one or two turns of DNA helix accordingly called one turn or two turn signal sequences Class switching is a process by which on antigenic stimulation, the H chain VDJ unit can join any constant heavy gene segment and express the paicular antibody class. Reference : Textbook of Microbiology (Ananthanarayanan and Paniker's) - pg 101 Introduction to Immunology (C. Vaman Rao) - pg 148,149 Springer.com Nature.com NCBI