Summary.
Immunocytomas are defined as disorders in the B‐lymphocyte series, characterized by production of an M‐component. After a basic description of the immunoglobulin molecule and the development of the cellular immune system by which it is built up, the production of M‐components is discussed in relation to possible aetiology and pathogenesis. The turn‐over of M‐components is mentioned, especially on the basis of the studies of Salmon and colleagues (Salmon, 1973) with reference to the different types of M‐components and to cellular aspects. The production of interferon and osteoclast activating factor is touched on, and a brief review is given of the most common clinical features of malignant immunocytomas. Finally, the interrelationship of the different manifestations of immunocytomas is described on the basis of the hypothesis put forward by Salmon & Seligmann (1974).