The basic structure of immunoglobulin (Ig) [1] 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 [2] 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 [3], 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.