The basic structure of immunoglobulin (Ig)  molecules is a tetramer of two
light chains and two heavy chains linked by disulfide bonds. There are two
types of light chains: kappa and lambda, each composed of a constant domain
(CL) and a variable domain (VL). There are five types of heavy chains: α,
delta, epsilon, γ and mu, all consisting of a variable domain (VH) and
three (in α, delta and γ) or four (in epsilon and mu) constant
domains (CH1 to CH4).
The major histocompatibility complex (MHC) molecules are made of two chains.
In class I  the α chain is composed of three extracellular domains, a
transmembrane region and a cytoplasmic tail. The β chain (β-2-microglobulin) is composed of a single extracellular domain. In class II ,
both the α and the β chains are composed of two extracellular domains,
a transmembrane region and a cytoplasmic tail.
It is known [4,5] that the Ig constant chain domains and a single
extracellular domain in each type of MHC chains are related. These
homologous domains are approximately one hundred amino acids long and
include a conserved intradomain disulfide bond. We developed a small pattern
around the C-terminal cysteine involved in this disulfide bond which can be
used to detect these category of Ig related proteins.
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