A considerable amount is known about Ig biosynthesis by mature plasma cells, which form large amounts of Ig for secretion from the cell. A brief summary is given of the formation of light (L) and heavy (H) chains by polyribosomes aligned on the endoplasmic reticulum and the rapid assembly of the chains into 7S molecules (H2L2) by disulphide bonding. There is a time-ordered secretion from the cell of 7S Ig molecules; the polymeric forms of Ig, ie, IgM and IgA, are formed from monomers by disulphide bond interchange and J chain incorporation at the time of secretion. Myeloma cells from mouse and man have proved very useful in this type of study but such malignant cells show many defects in regulatory mechanisms; therefore, no conclusions can be drawn about normal control mechanisms without analysis of lymphoid tissues from normal or immunized animals. The pattern of Ig synthesis by the mature cell contrasts with that by small B lymphocytes which form 1/50 to 1/100 the amount of Ig produced by mature cells. Most of the small lymphocyte Ig is associated with the cell surface, and in IgM-producing cells the surface receptors are 7S monomer subunits of IgM. Such receptors turn over slowly (24-48 hours); they may be gradually shed from the cell surface but the small lymphocyte does not actively secrete Ig. Antigen- and cell-cell interactions stimulate small B lymphocytes to divide and mature into Ig-secreting cells. Little is known about the associated intracellular events, but preliminary data on lipopolysaccaride-stimulated mouse spleen cells indicate that transcription of m-RNA for H-chain mirrors the kinetics of DNA synthesis. A translational block then occurs during cell maturation and there is a lag of at least 24 hours before Ig production rises sharply and reaches peak levels.
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