A protein produced by plasma cells and lymphocytes and characteristic of these types of cells. Immunoglobulins play an essential role in the body's immune system. They attach to foreign substances, such as bacteria, and assist in destroying them. Immunoglobulin is abbreviated Ig. The classes of immunoglobulins are termed immunoglobulin A (IgA), immunoglobulin G (IgG), immunoglobulin M (IgM), immunoglobulin D (IgD) and immunoglobulin E (IgE). Membrane-bound immunoglobulins are associated non-covalently with two accessory peptides, forming the B-cell antigen receptor complex. The first antigen receptors expressed by B cells are IgM and IgD. The receptor is a prototype of the antibody that the B cell is prepared to produce. The B cell receptor (BCR) can only bind antigens. It is the heterodimer of Ig alpha and Ig beta that enables the cell to transduce the signal and respond to the presence of antigens on the cell surface. The signal generated causes the growth and proliferation of the B cell and antibody production inside the plasma cell.