BONE MARROW IS A MAJOR SITE OF ANTIBODY SYNTHESIS
Although B‐cells mature in the bone marrow from hematopoietic stem cells, upon maturation most naive B‐cells leave for the secondary lymphoid tissues where they can encounter antigen. This release from the bone marrow may be regulated by sphingosine 1‐phosphate, which is known to control the exit of lymphocytes from the thymus and lymph nodes.
Figure 6.17 Plasma cells in human bone marrow. Cytospin preparation stained with rhodamine (orange) for IgA heavy chain and fluorescein (green) for lambda light chain. Both images are of the same field, one showing the green fluoresence, the other the orange fluorescence. Thus, one cell is IgA.λ, another IgA.non‐λ and the third is non‐IgA.λ
Activated B‐cells can recirculate back to the bone marrow and cluster around vascular sinusoids. In this location they are able to partake in the generation of antibody responses to blood‐borne pathogens and their survival is maintained by bone marrow dendritic cells secreting the cytokine MIF (macrophage migration inhibitory factor). The bone marrow is known to be the major residence of long‐lived plasma cells (Figure 6.17), the precursors of which are generated in the germinal centers of secondary lymphoid tissues. Thus, the bone marrow is a major source of serum Ig. Both B and T memory cells are also present.