Heavy-chain diseases (HCDs) are lymphoproliferative disorders of B cells characterized by the production of a monoclonal immunoglobulin (Ig) characterized by truncated heavy (H) chains and no associated light (L) chains. The diagnosis of these conditions depends on the detection of the structurally abnormal immunoglobulin molecules in the patients' serum or urine. Heavy-chain diseases involving the three main immunoglobulin classes have been described: ⍺-HCD is the most frequent, μ-HCD is rare, and γ-HCD is of intermediate incidence.