The bone marrow: the major source of serum immunoglobulins, but still a neglected site of antibody formation.

Immunoglobulin (Ig) secreting cells occur in all lymphoid tissues, including the bone marrow (BM). There are important differences between the various organs with respect to their number of Ig-secreting cells and the heavy chain isotype distribution of the secreted Igs. Furthermore, both distribution patterns depend on age. Early in life most Ig-secreting cells are localized in spleen and lymph nodes. In adults, however, the majority of all Ig-secreting cells of the individual are localized in the BM. Immunization can lead to the appearance of substantial numbers of antibody-forming cells in BM. The kinetics of the BM response are different from the response in the peripheral lymphoid tissues. Shortly after immunization most antibody-forming cells occur in the peripheral lymphoid tissues, but later on, especially during secondary type responses, most antibody-forming cells are localized in the BM. Apparently, antibody formation is regulated in such a way that peripheral lymphoid tissues respond rapidly, but only for a short period, whereas the BM response starts slowly, but takes care of a long-lasting massive production of antibodies to antigens which repeatedly challenge the organism.