After the contact with an antigen, B-lymphocyte differentiates into the plasma cell through the process of blastic transformation.
In this process the cell passes the following intermediate stages:

During the maturation the change of morphology B-lymphocytes is paralleled by changes in functional characteristics as well, producing finally the plasma cell that acquire an ability of producing and of secreting antibodies, i.e., serumal immunoglobulins. The production of specific antibodies is a basic function of the immune humoral response.
Most immunoglobulins are secreted by plasma cells – finally differentiated, i.e., mature part of B-lymphocytic system, while a small amount of immunoglobulins is secreted by directed predecessors of plasma cells.
They are committed to the synthesis, assembly, and secretion of immunoglobulin and have a well- developed rough endoplasmic reticulum and Golgi’s complex. They lack many of the surface receptors found on lymphocytes. Mature plasma cells are finally differentiated and have a low rate of DNA synthesis.
Automatic cell counters do not register them as a special type of cells in the system of differential blood picture.
The cells of the plasmocytic lineage can be found in the smears of the bone marrow, punctates of the lymph nodes and the spleen but not in the blood smears of healthy people.