Explain the mechanisms that induce activation of B cells to proliferate and secrete antibodies?

Only cells that express Igm heavy chain are selected to survive to become pre-B cells. Igm heavy chain and surrogate light chains are expressed on cell surface and associate with Iga and Igb to form pre-BCR complex. Pre-BCR complex is a first checkpoint for cells that express functional Igm heavy chain. It sends survival signals. If Igm heavy chain is failed to be made – pre-B cells will die by apoptosis= first checkpoint. Allelic exclusion Pre-BCR complex also send signal to inhibit recombination on the homologous chromosome, so only one allele will express BCR of single specificity. If k light chain fail to produce functional receptor or k light chain encodes a self-reacting BCR, then BCR will trigger recombination of k with l light chain. If functional IgM receptor is produced it will send survival signal, otherwise – apoptosis. Signals from IgM will also shut off light chain recombination, so either k or l light chains are made by given B cells. Signaling by functional IgM is a second checkpoint. B lymphocyte expressing IgM is the immature B cell. Further maturation occurs in bone marrow or in spleen. Heavy chain is splices into Cm or Cd. As a result, IgM and IgD are expressed by the same cells.