In most plasma cell disorders, monoclonal proteins have a structure similar to normal molecules but in abnormal amounts. Heavy chain disease (HCD) is a rare pathology defined by the presence of a monoclonal immunoglobulin (Ig) of abnormal structure, consisting only of an incomplete fragment of the heavy chain (truncated) and devoid of light chains, remarkable for the frequency of associated immunological or haematological diseases.Abnormal lymphocytes or plasma cells secrete the different types of heavy chains (alpha [α], gamma [γ], mu [μ] or delta [δ]) without light chains. The majority of heavy chain proteins are fragments of their normal counterparts with internal deletions of variable length. These deletions seem to result from structural mutations.There are 4 types of MCL according to the Ig heavy chain class produced by clonal cells: α, γ, μ or δ. The clinical presentation is closer to that of lymphoma than to multiple myeloma.The diagnosis of HCD and related diseases is based on the histological study of affected tissues and the analysis of serum or urine. The histological analysis with immunostaining shows a positive monoclonal population for a heavy chain, but negative for the two kappa (κ) and lambda (λ) light chains.The frequency of detection of the monoclonal peak is variable and depends on the type of HCD. The prognosis of these diseases is very variable from one patient to another and the treatment is not standardized.