Overview of plasma cell disorders

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Plasma cell disorders are uncommon. They begin when a single plasma cell multiplies excessively. The resulting group of genetically identical cells (called a clone) produces a large quantity of a single type of antibody (immunoglobulin). Plasma cells develop from B cells (B lymphocytes), a type of white blood cell that normally produces antibodies. These proteins help the body fight infection.
Plasma cells are present mainly in bone marrow and lymph nodes. Every plasma cell divides repeatedly to form a clone. The cells of a clone produce only one specific type of antibody. Because thousands of different clones exist, the body can produce a vast number of different antibodies to fight the numerous infectious microorganisms to which the body is exposed.
In plasma cell disorders, one clone of plasma cells multiplies uncontrollably. As a result, this clone produces vast amounts of a single antibody (monoclonal antibody) known as the M-protein. In some cases (such as with monoclonal gammopathies), the antibody produced is incomplete, consisting of only light chains or heavy chains (functional antibodies normally consist of two pairs of two different chains called a light chain and heavy chain).
The abnormal plasma cells and the antibodies they produce are limited to one type, and levels of other types of antibodies that help fight infections fall. Thus, people with plasma cell disorders are often at higher risk of infections. The ever-increasing number of abnormal plasma cells also invades and damages various tissues and organs, and the antibody produced by the clone of plasma cells can sometimes damage vital organs, especially the kidneys and bones.
Plasma cell disorders from more common to less common include

Monoclonal gammopathy of undetermined significance
Multiple myeloma
Macroglobulinemia (rare)
Heavy chain diseases (rare)

These disorders are more common among older people.