It recognizes an intra-cytoplasmic antigen, which shows a very high degree of specificity for plasma cells. This antigen is present in normal as well as neoplastic plasma cells. Plasma cells, which are large lymphocytes derived from an antigen-specific B cell, secrete antibodies and are responsible for humoral immunity. Plasma cells differentiate from B cells upon stimulation by CD4+ lymphocytes. The B cell acts as an antigen-presenting cell (APC), consuming an offending pathogen, which is taken up by the B cell by phagocytosis and broken down within proteosomes. Plasma cells contain basophilic cytoplasm; their nucleus contains heterochromatin organized in a characteristic cartwheel arrangement. This MAb superbly recognizes normal and neoplastic plasma cells in routine formalin-fixed, paraffin-embedded tissue sections. It is of potential value in identifying myeloma or plasmacytoma in bone marrow or other tissues. It also helps differentiate lympho-plasmacytoid lymphoma from lymphocytic and follicular lymphoma. Note that this MAb is not suitable for staining frozen tissues.