Plasma cells arise from B cells and are thus components of the immune system. This cell form is a terminal stage of B cells that are no longer able to divide and is capable of producing antibodies. In diseases such as multiple myeloma, degenerate plasma cells proliferate in a malignant manner.
What are plasma cells?
Plasma cells are blood cells that are also called mature B lymphocytes. Like T lymphocytes, they are part of the immune system. All lymphocytes are white blood cells, or leukocytes, that play a role primarily in the immune response. They produce and secrete antibodies. As so-called effector cells, they are the product of the final differentiation stage of the B cell line. Unlike B cells, plasma cells are no longer capable of division. They migrate via the blood into the bone marrow, where they are supplied by the stromal cells. They continue the production and secretion of antibodies there. After the last division, a part of the B lymphocytes becomes so-called B memory cells, which are relevant for the immunological memory and thus the learning ability of the human immune system. Plasma cells are formed from the B lymphocytes that have not turned into memory cells after the last division. The function of plasma cells was first described by immunologist Astrid Fagraeus in the 20th century.
Anatomy and structure
Plasma cells are activated B cells. Their activation is due to contact with a specific antigen. Via the stage of plasmablast, the B cell has become the plasma cell after activation. The cells are roundish to oval in shape. They have a diameter of ten to 18 µm. Because of this small diameter, they can travel in the thinnest ramifications of the bloodstream. Instead of being granular, their cytoplasm is basophilic. This final form of B cells contains a relatively large amount of cytoplasm. Due to numerous layers of the endoplasmic reticulum, plasma cells can synthesize a particularly large number of antibodies. In an eccentric position, they possess a so-called wheel storage nucleus. Because, unlike their predecessors, they do not have MHC-II, they do not present any signals to the T helper cells. Instead, they still express surface immunoglobulins in small numbers.
Function and Tasks
B cells represent a specific antigen. When these cells encounter specialized T helper cells in the lymph nodes whose specialization matches their antigen representation, B cell activation occurs. Such an encounter can only occur after direct contact with a specific antigen. In this way, the B cells become plasma cells that produce antibodies themselves. Some of these plasma cells go back to the primary lymphoid follicles. There they form the germinal center. However, plasma cells can only form the germinal center if they have been activated by a T cell. When activated independently of T cells, B cells do not undergo an isotype change. They produce only IgM-type antibodies and cannot develop into memory B cells. B cells in the germinal center change isotype and become plasma cells that produce high affinity antibodies in different classes of immunoglobulins. A proportion of these cells become memory B cells, through which information about the specific antigens is provided to the organism. Since the memory cells remember the first contact when they encounter an antigen again, they can be activated more quickly and provide a more effective immune response. Plasma cells with high affinity antibodies from different classes make their way to the bone marrow. There they are supplied by the stromal cells and can thus release antibodies for a certain time. By their respective expression, human plasma cells can be characterized by the surface markers CD19, CD38, and CD138.
Diseases
The best-known disease of plasma cells is multiple myeloma, also known as plasmacytoma. In multiple myeloma, plasma cells degenerate and malignant proliferation occurs. This disease is a cancer of the bone marrow. Normally, the degenerated cells still produce antibodies in fragments. The antibodies are absolutely identical to each other. The course of the disease can be very different.While some forms of this disease can only be characterized as precancerous, others are highly malignant and are usually lethal in a very short time. Bone pain, bone fractures and a slow dissolution of the bone substance by the malignant antibodies are the most important symptoms. Serum calcium is elevated and red blood cells are decreased. The degenerated antibodies are deposited in the organs and tissues and can cause manifestations such as kidney failure. Apart from diseases affecting the plasma cells themselves, the plasma cell count can signal various other diseases and conditions to the physician. In chronic alcohol abuse, for example, excessively high levels can be detected in the serum. In the case of syphilis of the large lymphatic vessels, on the other hand, the concentration of plasma cells is lowered. Presumably, IgG4-associated diseases are also related to plasma cells. This is either an autoimmune disease or an allergic reaction. The disease has not yet been conclusively investigated. However, the proliferation of IgG4-positive plasma cells in organ tissue could be observed as a disease criterion. The affected organ then becomes inflamed and nodular changes develop, which are triggered by fibrosis. In most cases, severe fever is present concomitant with these manifestations.
