Heavy chain disease is a form of paraproteinemia with a proliferation of cells producing immunoglobulin heavy chains.
This disease is characterized by an excessive production of heavy chains that are short and truncated. These heavy chain disease proteins have various deletions, mainly in their amino-terminal part, which causes the heavy chains to lose the ability to form disulfide bonds with the light chains. The defect in the immunoglobulins presumably arises during somatic hypermutation.Deletion of the N-terminal part of the heavy chain disease protein leads to aggregation and signaling of the B cell receptor presumably due to the loss of the anti-aggregating properties of the light chain.
Classification
There are four forms:

• alpha chain disease (Seligmann's disease)
• gamma chain disease (Franklin's disease)
• mu chain disease
• delta chain disease

IgA/αHCD
The most common type of heavy chain disease is the IgA type, known as αHCD. The most common type of αHCD is the digestive form, however it has also been reported in the respiratory tract, and other areas of the body.
IgG/γHCD
Franklin's disease (gamma heavy chain disease) It is characterized by lymphadenopathy,fever,anemia ,malaise,hepatosplenomegaly and weakness. Most distinctive symptom is palatal edema,caused by nodal involvement of Waldeyer's ring. Diagnosis is made by the demonstration of an anomalous serum M component, that reacts with anti-IgG but not anti-light chain reagents. Bone marrow examination is usually nondiagnostic. Patients usually have a rapid downhill course and die of infection.
IgM and IgG heavy chain diseases, which are known as μHCD and γHCD respectively, are fairly common and are present in various tissues.
The γHCD can be divided into three categories based on the various clinical and pathological features. These categories are disseminated lymphoproliferative disease, localized proliferative disease and no apparent proliferative disease.

• Disseminated lymphoproliferative disease is found in 57-66% of patients diagnosed with γHCD. Lymphadenopathy and constitutional symptoms are the usual features.

• Localized proliferative disease is found in approximately 25% of γHCD patients. This is characterized by a localization of the mutated heavy chains in extramedullary tissue, or solely in the bone marrow.

• No apparent proliferative disease is seen in 9-17% of patients with γHCD, and there is almost always an underlying autoimmune disorder in these patients.

IgM/μHCD
The IgM type of heavy chain disease, μHCD, is often misdiagnosed as chronic lymphoid leukemia (CLL) because the two diseases are often associated with each other and show similar symptoms.