Multiple myeloma with monoclonal free IgG3 heavy chains and free kappa light chains

We describe the case of a 34-year-old gentleman investigated for persistent neutropaenia following two episodes of pneumonia. Specialist investigations led to the diagnosis of multiple myeloma (MM) producing a truncated monoclonal gamma(3) heavy chain (HC) immunoglobulin molecule unattached to a light chain (LC) with atypical features for both MM and HC disease. Western blot showed gamma(3)HC was truncated with a large deletion (75 kDa). Flow cytometry of the bone marrow aspirate revealed an unusual staining pattern. This plasma cell dyscrasia was also unusual in that a subpopulation (30%) secreted large quantities of free LC (FLC) as well as truncated IgG HC. This is the first description, investigation and treatment of MM with a plasma cell population producing truncated gamma(3)HC and kappaFLC M-proteins and illustrates a number of unique immunological and clinical features.