Heavy chain disease

Heavy chain disease is a form of paraproteinemia with a proliferation of heavy chains.

This disease is characterized by an excessive production of immunoglobulin heavy chains that are short and truncated. The amino acid sequence on the amino terminal is completely normal, but there is a deletion in the protein structure that extends from the middle of the variable region, through the first domain of the constant region, and ending just before the position of the first disulfide bond between the two heavy chains. This deletion causes the heavy chains to lose the ability to form disulfide bonds with the light chains. The defect in the immunoglobulins arises during the faulty coupling of the variable and constant regions during somatic recombination.

Classification
There are three forms:
 * alpha chain disease
 * gamma chain disease
 * mu chain disease

IgA/αHCD
The most common type of heavy chain disease is the IgA type, known as αHCD. This is because the most common immunoglobulin in the body is IgA. 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
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.