In humans, antibodies are composed of two identical heavy chains which are of the gamma (γ), alpha (α), epsilon (ε), delta (δ), or mu (μ) subtypes and two identical light chains which are of the kappa (κ) or lambda (λ) subtypes. Antibodies are classified as IgG, IgA, IgE, IgD, and IgM based on their being made up of γ, α, ε, δ, or μ heavy chains, respectively. Formation of the genes that make these antibodies requires B cells and/or their descendent plasma cells to mutate, break, and recombine various genes at the immunoglobulin heavy chain antigen-binding locus on the long (i.e. "q") arm of human chromosome 14 at position 32.33 (notated as 14q32.33) and the immunoglobulin light chain antigen binding locus on the q arm of chromosome 22 at position 11.2 (i.e. 22 q11.2) by processes termed V(D)J recombination, somatic hypermutation, and immunoglobulin class switching. These genomic changes can go awry by placing a gene that controls cell growth an/or survival adjacent to a normally highly active antibody gene promoter and/or by causing the formation of extra chromosomes (see trisomy) or chromosomes with large deletions that result in the overexpression or under-expression, respectively, of genes that control cell growth and/or survival.
Heavy chain disease
The four heavy chain diseases are exceedingly rare conditions associated with the production, circulation in blood, and often presence in urine of  a free clonal heavy chain with no detected clonal light chains. The heavy chain is non-functional and altered by having deletions, insertions, and point mutations due to somatic mutations in their respective coding genes. Beyond this commonality, however, these diseases have very different clinical differences. Furthermore, each of the heavy chain diseases appears to be due to rare variants of lymphoma and therefore is sometimes regarded as a B cell dyscrasia However, heavy chain diseases are still often classified with plasma cell dyscrasias. The heavy chain diseases are classified as α, γ, and μ heavy chain diseases and are based respectively on >400, 130, and 30-40 case reports as reviewed in a 2014 publication. α Heavy chain disease
α Heavy chain disease (also termed immunoproliferative small intestinal disease or IPSID, Mediterranean lymphoma, and Seligmann disease) afflicts primarily individuals of  Mediterranean, North African, and Middle Eastern descent of lower economic status. Many cases are centered in the Middle East and associated with relatively unsanitary living conditions. The disease usually appears between the ages 10 and 30 and in some cases may be an aberrant immune response to a parasite or other microorganism. The disease commonly effects the gastrointestinal tract leading to signs and symptoms of a malabsorption syndrome or, far less commonly, the respiratory tract with signs and symptoms of respiratory dysfunction.