Introduction

Plasma cell disorders (plasma cell dyscrasias) are uncommon. They begin when a single group (clone) of plasma cells multiplies excessively and produces a large quantity of abnormal antibodies. Plasma cells develop from B lymphocytes, a type of white blood cell, and normally produce antibodies (immunoglobulins), which help the body fight infection. Plasma cells are found mainly in bone marrow and lymph nodes. Every plasma cell divides repeatedly to form a clone, composed of many identical cells. The cells of a clone produce only one specific type of antibody. Thousands of different clones exist; thus, the body can produce a vast number of different antibodies (see Section 16, Chapter 183) to fight the body's frequent exposure to infectious microorganisms.

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). These 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.

Plasma cell disorders include monoclonal gammopathies of undetermined significance, multiple myeloma, macroglobulinemia, and heavy chain diseases. These disorders are more common among older people.