Immunoglobulin light chains that are circulating in serum in a free (unbound) state are called free light chains (FLCs). Measurement of the serum level of FLCs became practical as a clinical blood test in recent decades. These tests are used as an aid in the diagnosis and monitoring of multiple myeloma and related disorders. There are two types of immunoglobulin light chain produced in humans, designated by the Greek letters kappa (κ) and lambda (λ). Comparing the ratio of κ FLCs to λ FLCs in a person's serum against reference ranges indicates whether that person may have a plasma cell tumour such as multiple myeloma or AL amyloidosis. Structure Each immunoglobulin light-chain molecule contains approximately 220 amino acids in a single polypeptide chain that is folded to form constant and variable region domains. Each domain comprises two β-pleated sheets. The sheets are linked by a disulfide bridge and together form a roughly barrel-shaped structure known as a β-barrel. The variable (V) domain of light chains has a high degree of structural diversity, particularly the antigen-binding region. In addition, the first 23 amino acids of the 1st variable domain framework region have a number of variations known as subgroups. Four kappa (Vκ1–Vκ4) and six lambda subgroups (Vλ1–Vλ6) can be identified. The specific subgroup structures influence the potential of the free light chains to polymerise such that AL amyloidosis is associated with Vλ6 and light-chain deposition disease with Vκ1 and Vκ4. Synthesis Kappa light-chain molecules are constructed from approximately 40 functional Vκ gene segments (chromosome 2), five Jκ gene segments and a single Cκ gene. Lambda molecules (chromosome 22) are constructed from about 30 Vλ gene segments and four pairs of functional Jλ gene segments and a Cλ gene. Light chains are incorporated into immunoglobulin molecules during B-cell development and are expressed initially on the surface of pre B-cells. Production of light chains occurs throughout the rest of B-cell development and in plasma cells, where secretion is highest. Production The production of free immunoglobulin light chains in normal individuals is approximately 500 mg/day from bone marrow and lymph node cells. There is approximately 40% excess immunoglobulin light-chain production over immunoglobulin heavy-chain synthesis. Possibly this is simply to allow proper conformation of the intact immunoglobulin molecules, but an immunological role for the free light chains has also been proposed. There are approximately twice as many kappa-producing plasma cells as lambda plasma cells. Kappa