CD138 / syndecan-1 protein backbone is a single chain molecule of 30.5 kDa. Five putative GAG attachment sites exist in the extracellular domain. GAG fine structure appears to reflect the cellular source of the syndecan. Expression of CD138 antibody in human hematopoietic cells is restricted to plasma cells in normal bone marrow. Early B-cell precursors in human bone marrow are CD138 negative. CD138 may aid in distinguishing between viable myeloma cells vs. apoptotic cells. CD138 is also expressed in endothelial cells, fibroblasts, keratinocytes and normal hepatocytes.
Antigen detection in tissues and cells is a multi-step immunohistochemical process. The initial step binds the primary antibody to its specific epitope. After labeling the antigen with a primary antibody, a secondary antibody is added to bind to the primary antibody. An enzyme label is then added to bind to the secondary antibody; this detection of the bound antibody is evidenced by a colorimetric reaction.