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.

There are two types of light chain in humans (as in other mammals),

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 3:1 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, according to a novel polyclonal free light chain assay, 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.

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 also 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.

Sharks also possess, as part of their adaptive immune systems, a functional heavy-chain homodimeric antibody-like molecule referred to as IgNAR (immunoglobulin new antigen receptor). IgNAR is believed to have never had an associated light chain, contrary to the understanding that the heavy-chain-only antibodies in camelids may have lost its light chain partner through evolution.

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

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