Immunoglobulin light chain

The immunoglobulin light chain is the small polypeptide subunit of an antibody (immunoglobulin).

A typical antibody is composed of two immunoglobulin (Ig) heavy chains and two Ig light chains.

In humans
There are two types of light chain in humans (as in other mammals),
 * kappa (κ) chain, encoded by the immunoglobulin kappa locus (IGK@) on chromosome 2
 * lambda (λ) chain, encoded by the immunoglobulin lambda locus (IGL@) on chromosome 22

Antibodies are produced by B lymphocytes, each expressing only one class of light chain. Once set, light chain class remains fixed for the life of the B lymphocyte. In a healthy individual, the total kappa to lambda ratio is roughly 2:1 in serum (measuring intact whole antibodies) or 1:1.5 if measuring free light chains, with a highly divergent ratio indicative of neoplasm.

The exact normal ratio of kappa to lambda ranges from 0.26 to 1.65. Both the kappa and the lambda chains can increase proportionately, maintaining a normal ratio. This is usually indicative of something other than a blood cell dyscrasia, such as kidney disease.

In other animals
The immunoglobulin light chain genes in tetrapods can be classified into three distinct groups: kappa (κ), lambda (λ), and sigma (σ). The divergence of the κ, λ, and σ isotypes preceded the radiation of tetrapods. The σ isotype was lost after the evolution of the amphibian lineage and before the emergence of the reptilian lineage.

Other types of light chains can be found in lower vertebrates, such as the Ig-Light-Iota chain of Chondrichthyes and Teleostei.

Camelids are unique among mammals as they have fully functional antibodies which have two heavy chains, but lack the light chains usually paired with each heavy chain. The functional role of this separate repertoire is unknown as yet.

Structure
Only one type of light chain is present in a typical antibody, thus the two light chains of an individual antibody are identical.

Each light chain is composed of two tandem immunoglobulin domains:
 * one constant (CL) domain
 * one variable domain (VL) that is important for binding antigen

The approximate length of a light chain protein is from 211 to 217 amino acids.

In pathology
Ig light chains produced in neoplastic plasma cells, e.g. in multiple myeloma, are called Bence Jones proteins.